allow Models and BitmapTexts in BGAnimationLayer (detected through file extension)
add RageModelVertex, which doesn't have a color property
add "stretch" file name hint for textures
UpdateTexture can now take any image format; it doesn't have to match the
original format, or even be a standard pixel format. This is because we
can often do the conversion much more efficiently within UpdateTexture
than we can in the caller. With D3D, we can convert any source format
as we copy, preventing an extra blit. OpenGL can do it on the fly if
it happens to be a known texture format.
We can figure out the pixel format from the image itself, so there's
no need to pass pixfmt to UpdateTexture.
Add BGR8, for movies with OpenGL.
Chris Danford