fix printing Failed to determine valid GL format

[huanghongxun]

This PR fixes incorrect treatment to return value of epoxy_gl_version()

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[huanghongxun]

@robert-ancell

[cbracken]

lgtm

[robert-ancell]

@huanghongxun I just noticed this is checking for the GL_EXT_bgra extension before setting GL_RGB8 - is this a mistake? Should the check be inverted?