Test either GL_FRONT_LEFT or GL_FRONT for front-buffer rendering
Test either GL_FRONT_LEFT or GL_FRONT for front-buffer rendering

For non-stereo visuals, which is all we support, we treat
GL_FRONT_LEFT as GL_FRONT.  However, they are technically different,
and they have different enum values.  Test for either one to determine
if we're in front-buffer rendering mode.

This fix was suggested by Pierre Willenbrock.

Signed-off-by: Ian Romanick <2b237cafb16dc45038e85df6c85e74e6d899eba9@intel.com>

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