Recreate context if sRGB framebuffer required

If an sRGB framebuffer is required the current code cannot always identify the correct pixel format to use, as without an existing context the required extensions are not available.

This patch adds a test to the existing _glfwAnalyzeContext function, following the same format as the existing test for FSAA.

Note that under most circumstances implementations which can offer sRGB buffers do so on pixel formats which can support sRGB. Thus this test doesn't seem to be practically useful, so there is some question as to whether to add the extra context creation required.