vrend: deal with BGRX upload on GLES
If the texture upload doesn't use GBM we end up with trying to upload a RGBA texture to a resources that had the internal format RGB8, so work around this by setting the format as GL_RGB8 and not only swizzle the input, but also remove the extra byte that holds the un-used alpha channel.
This patch doesn't handle passing sRGB data, that is, the texture upload should not fail, but the colors will be passed as if they were linear.