Split out some compiler-version-specific definitions into "compiler.h".

Split out 'getpixel' and 'putpixel' family of functions into "pixels.c" and "pixels.h" source files.

src/compiler.h

@@ -0,0 +1,57 @@
+#ifdef __GNUC__
+// This version has strict type checking for safety.
+// See the "unnecessary" pointer comparison. (from Linux)
+#define min(x,y) ({ \
+  typeof(x) _x = (x);     \
+  typeof(y) _y = (y);     \
+  (void) (&_x == &_y);            \
+  _x < _y ? _x : _y; })
+#define max(x,y) ({ \
+  typeof(x) _x = (x);     \
+  typeof(y) _y = (y);     \
+  (void) (&_x == &_y);            \
+  _x > _y ? _x : _y; })
+#else
+#define min(a,b) (((a) < (b)) ? (a) : (b))
+#define max(a,b) (((a) > (b)) ? (a) : (b))
+#endif
+
+#define clamp(lo,value,hi)    (min(max(value,lo),hi))
+
+
+// since gcc-2.5
+#ifdef __GNUC__
+#define NORETURN __attribute__((__noreturn__))
+#define FUNCTION __attribute__((__const__))  // no access to global mem, even via ptr, and no side effect
+#else
+#define NORETURN
+#define FUNCTION
+#endif
+
+#if !defined(restrict) && __STDC_VERSION__ < 199901
+#if __GNUC__ > 2 || __GNUC_MINOR__ >= 92
+#define restrict __restrict__
+#else
+#warning No restrict keyword?
+#define restrict
+#endif
+#endif
+
+
+#if __GNUC__ > 2 || __GNUC_MINOR__ >= 96
+// won't alias anything, and aligned enough for anything
+#define MALLOC __attribute__ ((__malloc__))
+// no side effect, may read globals
+#define PURE __attribute__ ((__pure__))
+// tell gcc what to expect:   if(unlikely(err)) die(err);
+#define likely(x)       __builtin_expect(!!(x),1)
+#define unlikely(x)     __builtin_expect(!!(x),0)
+#define expected(x,y)   __builtin_expect((x),(y))
+#else
+#define MALLOC
+#define PURE
+#define likely(x)       (x)
+#define unlikely(x)     (x)
+#define expected(x,y)   (x)
+#endif
+