Completed Negative Magic tool as a plugin (except sound).

Created Mirror and Flip Magic tools as plugins (except sound); hooks
added to Magic plugin API to allow starter and starter-undo mirror/flip.
Created Fade and Darken Magic tools as plugins.
Moved magic tool sounds to 'magic'.
Added flip and mirror icons to main Tux Paint UI (used to share magic tools')
Created 'tp-magic-config.sh' to get CFLAGS for compiling magic tool plugins.
Improved magic tool plugin API.
Committed some documentation on magic tool plugin API.

magic/docs/README.txt

@@ -0,0 +1,131 @@
+Stub docs for Tux Paint Magic Plugin Development
+
+#include "tp_magic_api.h"
+
+
+build plugins with:
+-------------------
+  Linux/Unix:
+  -----------
+    $(CC) -shared `tp-magic-config --cflags` plugin.c -o plugin.so
+
+    Then install globally into:  /usr/[local/]lib/tuxpaint/.
+    Or locally into:  ~/.tuxpaint/magic/  [[FIXME]]
+
+  Windows:
+  --------
+    ??? [[FIXME]]
+
+  Mac OS X:
+  ---------
+    ??? [[FIXME]]
+
+
+magic plugins must provide:
+---------------------------
+  NOTE: Each function name should be preceded with the name of the
+  shared object file, e.g. "negative.so" or "negative.dll" would have
+  a function called "negative_init()".
+
+  int get_tool_count(magic_api * api)
+    return the number of Magic tools this plugin provides
+    (used below as the 'which' values sent to each function)
+
+  char * get_name(magic_api * api, int which)
+    return the name of a/the magic tool (for 'Magic' tool buttons in UI)
+    Tux Paint will free() the string;
+    example:
+      return (strdup(gettext("Fun")));
+
+  SDL_Surface * get_icon(magic_api * api, int which)
+    return the icon of a/the magic tool (for 'Magic' tool buttons in UI)
+    Tux Paint will SDL_FreeSurface() the surface:
+    example:
+      sprintf(fname, "%s/images/magic/funtool.png", api->data_directory);
+      return(IMG_Load(fname));
+
+  char * get_description(magic_api * api, int which)
+    return the description of a/the magic tool (for Tux help text in UI);
+    Tux Paint will free() the string;
+    example:
+      return (strdup(gettext("A fun tool")));
+
+  int requires_colors(magic_api * api, int which)
+    return whether a/the magic tool accepts colors
+    ('1' for true; activates color palette in UI;
+     '0' for false; disables color palette in UI)
+
+  void set_color(magic_api * api, Uint8 r, Uint8 g, Uint8 g)
+    accept the color palette choice from Tux Paint
+    (only called if requires_colors() for the current tool returned true)
+
+  int init(magic_api * api)
+    initialization function; called once, during Tux Paint startup
+    return 1 if success;
+    return 0 if failure (tool(s) will be disabled in Magic tool);
+
+  void shutdown(magic_api * api)
+    cleanup function; should free any alloc'd memory, etc.;
+    happens once, at Tux Paint shutdown
+
+  void click(magic_api * api, int which,
+             SDL_Surface * snapshot, SDL_Surface * canvas,
+             int x, int y)
+    should affect 'canvas' at (x,y) location; may use 'snapshot' to fetch
+    pixels from most recent undo buffer;
+    Tux Paint's undo buffer is updated prior to this call
+
+  void drag(magic_api * api, int which,
+            SDL_Surface * snapshot, SDL_Surface * canvas,
+            int ox, int oy, int x, int y)
+    should affect 'canvas' between (ox,oy) and (x,y) locations;
+    may use 'snapshot' to fetch pixels from most recent undo buffer;
+    Tux Paint's undo buffer is NOT updated prior to this call; the
+    'snapshot' buffer will contain the same contents as when click() was
+    last called
+
+
+tp provides, via magic_api structure ("api" arg to magic tool functions):
+-------------------------------------------------------------------------
+  void putpixel(SDL_Surface * surf, int x, int y, Uint32 pixel)
+    function that puts a pixel at an (x,y) position in a surface
+
+  Uint32 getpixel(SDL_Surface * surf, int x, int y)
+    function that returns a pixel value from an (x,y) position on a surface
+
+  int in_circle(int x, int y, int radius)
+    function that returns whether an x/y position (centered at (0,0)) is
+    within a circle of 'radius'
+
+  void show_progress_bar(void)
+    draws the Tux Paint progress bar animation; use while you're busy
+
+  void tuxpaint_version(int * major, int * minor, int * revision)
+    returns the version of Tux Paint being used
+
+  void line(int which, SDL_Surface * canvas, SDL_Surface * snapshot,
+            int x1, int y1, int x2, int y2, int step, FUNC callback)
+    function that calls calculates a line between (x1,y1) and (x2,y2)
+    and calls 'callback' every 'step' iterations;
+    sends to the callback: Tux Paint's 'magic_api' structure, along with the
+    'which', 'canvas' and 'snapshot' values sent to line(), and the (x,y)
+    coordinates
+
+
+FIXME: Implement these:
+
+  void playsound(Mix_Chunk * snd, int pan, int dist)
+    function that plays a sound, panned left/right 'pan'
+    and at distance 'dist'.  pan may be SNDPOS_LEFT, SNDPOS_CENTER or
+    SNDPOS_RIGHT, and dist may be SNDDIST_NEAR.
+
+  void special_notify(int flag)
+    notifies tux paint of special events; SPECIAL_FLIP and SPECIAL_MIRROR
+    flags may be sent
+
+  float sRGB_to_linear_table[256]
+    sRGB-to-linear look-up table
+
+  unsigned char linear_to_sRGB(float linear)
+    linear-to-sRGB look-up helper function
+