OpenGL: Fix erroneous handling of BGRX

BGRX was being treated as "BGR input" with "RGBA storage", where it
should have been "BGRA input" with "RGB storage".  So the input for the
texture was expecting 24 bits of packed BGR rather than 32bit BGRX
pixels, and was internally storing it with alpha available.
ignore test-random alpha (to ensure BGRX works)