Bindings for some missing parts of the raw SDL2 API

[evanrinehart]

Namely the functions SDL_ComposeCustomBlendMode and the associated enums,
SDL_GetWindowBordersSize, and SDL_GetDisplayUsableBounds.

I ran into an issue combining Cairo and SDL2 where I needed the custom blending mode. Cairo outputs texture data which uses pre-multiplied alpha, but SDL built-in blending modes do not support this. So blending two anti-aliased cairo-generated textures with SDL2 looks really bad by default, too dark around the edges. It works (on OSX) if you make a custom blending mode that implements the formulas

dstRGB = srcRGB + dstRGB*(1 - srcAlpha)
dstAlpha = srcAlpha + dstAlpha*(1 - srcAlpha)

Perhaps we're holding off on 2.0.6 support for some reason. Though I was surprised to see SDL_GetWindowBordersSize and SDL_GetDisplayUsableBounds missing, which were supposedly added in SDL 2.0.5.

Sorry I did not do a diff for all the missing API. These are just the things I ran into trying to make some graphics.

[ocharles]

Thanks for this! I'm a little behind on this project ATM, but I'll try and use this PR as a forcing function to get things up to date. Hopefully I'll merge this soon, it looks good!