Check for GL debug extensions before use

[JayFoxRox]

This change adds a check for support GL debug extensions, and then doesn't use the respective feature if they are not present.

This is to address reports of users who had ran into problems with glFrameTerminatorGREMEDY not being implemented on their system, even if the pointer was set (presumably to a wrapper like GLEW).

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• Wait for github support.. still
• Testing, this is entirely untested
• Wait for epoxy change

[JayFoxRox]

(work continues at JayFoxRox/xqemu)