We didn't correctly reject textures that would fail glTexImage2D with an GL_INVALID_OPERATION error. Trying to use the so accepted texture formats would later lead to errors.
Correct this error and also the definition of VIRGL_FORMAT_X24S8_UINT that is needed for GLES 3.1.
Closes: #15 (closed)
Signed-off-by: Gert Wollny firstname.lastname@example.org