Merge pull request #69 from dariomanesku/master

Added 15-shadowmaps-simple and 16-shadowmaps.
Updated shaders and README.
Updated screenshots.
Resized screenshots.
Cleanup.
GL: Disable scissor before FB blit.
Added imguiChoose function.
Merge branch 'master' of github.com:bkaradzic/bgfx
Replaced static const values with defines.
Swiched from FPU to SIMD math.
Added 17-drawstress example.
Fixed Linux build.
Cleanup.