Fixup the graphics library API

[klange]

The graphics library exports symbols without any prefix, leading to some problematically short names and a lack of uniformity. Functions like flip or rgb/rgba absolutely need some prefixing. The headers also include a number of problematic macros.

Thoughts:

• Prefix all the export symbols with gfx_.
  • init_graphics_fullscreen → gfx_init_fullscreen()
  • alpha_blend → gfx_alpha_blend, etc.
  • flip → gfx_context_flip
  • draw_* → gfx_draw_*
• Use longer macro names, GFX() → GFX_CTX_PIXEL(), SPRITE() → GFX_SPRITE_PIXEL(), etc.
• Rename sprite_t → gfx_sprite_t

[klange]

The published SDL package for 2.x references flip and some init_graphics_... functions, so it will need to be updated.

[akshataj96]

If this is still open, Can i start looking into it? @klange

[klange]

This is not the sort of thing I would normally accept a pull request for, and it is not practical for an external contributor anyway - much of the work here will be making changes to parches for ported third-party software (SDL, etc.), as well as repackaging it and updating the CDN.