Tweak video pixel format for faster OpenGL texture loading

Instead of sending a RGB buffer to the card, send a BGRA buffer.
The 32-bits alignment and the blue/red order are likely to be
native to the video card, avoiding costful conversions.

This combination is reported to best here:
http://www.opengl.org/wiki/Common_Mistakes#Image_precision
https://developer.apple.com/library/mac/documentation/graphicsimaging/conceptual/opengl-macprogguide/opengl_texturedata/opengl_texturedata.html#//apple_ref/doc/uid/TP40001987-CH407-SW1

On a Windows 7 PC with Intel HD Graphics card, timing the
glTexSubImage2d calls produces the following results:

-before the commit: Mean = 9.4 ms, standard dev = 1.5 ms
-after the commit:  Mean = 5.4 ms, standard dev = 1.6 ms
Merge pull request #219 from tlecomte/tlOpenGLTextureFormat2

Tweak video pixel format for faster OpenGL texture loading