Adding exception detection to render api.
- Bug in EnumerableConstants, last_const_value should have defaulted to
negative one.
- typedef :pixel_format changed from :uint32 to enum of
SDL2::PIXELFORMAT
- typedef :blend_mode changed from static enum to SDL2::BLENDMODE
- SDL2::Renderer generally improved and further implemented.
- SDL2::Texture further implemented with #create
- SDL2::Window touched but mostly the same..
Merge branch 'REBOOT' of github.com:BadQuanta/sdl2_ffi into REBOOT

Conflicts:
	lib/sdl2/render.rb
	spec/lib/sdl2/render_spec.rb

Think I kept the latest work.