WIP Rotational brushes now supported

Unlike "directional" brushes, in which a 3x3 grid representing the
8 cardinal directions (45 degree steps) is used, only a single brush
image is required, and Tux Paint will rotate it between 0 and 360 degrees,
depending on the direction the mouse is going.

The brush's ".dat" file should contain a line consisting of the word
"rotate".

Note: This adds a dependency on "SDL_gfx" library (Homepage:
https://www.ferzkopp.net/wordpress/2016/01/02/sdl_gfx-sdl2_gfx/
SourceForge project page: https://sourceforge.net/projects/sdlgfx/)
as this feature use it's "rotozoom" functionality.

WIP -- Doesn't handle animated brushes correctly yet!

Closes https://sourceforge.net/p/tuxpaint/feature-requests/122/

docs/ja_JP.UTF-8/INSTALL.txt

@@ -37,9 +37,10 @@ Requirements
        Lesser General Public License (LGPL).
 
        Along with libSDL, Tux Paint depends on a number of other SDL 'helper'
-       libraries: SDL_Image (for graphics files), SDL_TTF and (optionally)
-       SDL_Pango (for True Type Font support) and, optionally, SDL_Mixer (for
-       sound effects).
+       libraries: SDL_Image (for graphics files), SDL_gfx (for some graphical
+       functions, like rotation), SDL_TTF and (optionally) SDL_Pango (for
+       True Type Font support) and, optionally, SDL_Mixer (for sound
+       effects).
 
     Linux/Unix Users:
 
@@ -49,6 +50,9 @@ Requirements
 
            * libSDL: http://www.libsdl.org/
            * SDL_Image: http://www.libsdl.org/projects/SDL_image/
+           * SDL_gfx:
+             https://www.ferzkopp.net/wordpress/2016/01/02/sdl_gfx-sdl2_gfx/
+             (https://sourceforge.net/projects/sdlgfx/)
            * SDL_TTF: http://www.libsdl.org/projects/SDL_ttf/
            * SDL_Pango: http://sourceforge.net/projects/sdlpango/ (optional)
            * SDL_Mixer: http://www.libsdl.org/projects/SDL_mixer/ (optional)