Initial pixel shader support for YUV textures in the GL renderer.

This work is not complete yet!

--HG--
extra : convert_revision : svn%3Ac70aab31-4412-0410-b14c-859654838e24/trunk%403296

src/video/SDL_glfuncs.h

@@ -153,7 +153,7 @@ SDL_PROC_UNUSED(void, glGetPixelMapuiv, (GLenum map, GLuint * values))
 SDL_PROC_UNUSED(void, glGetPixelMapusv, (GLenum map, GLushort * values))
 SDL_PROC_UNUSED(void, glGetPointerv, (GLenum pname, GLvoid * *params))
 SDL_PROC_UNUSED(void, glGetPolygonStipple, (GLubyte * mask))
-SDL_PROC_UNUSED(const GLubyte *, glGetString, (GLenum name))
+SDL_PROC(const GLubyte *, glGetString, (GLenum name))
 SDL_PROC_UNUSED(void, glGetTexEnvfv,
                 (GLenum target, GLenum pname, GLfloat * params))
 SDL_PROC_UNUSED(void, glGetTexEnviv,