Fix BGRA/RGBA readback conversions on mac

Reading back from a BGRA framebuffer as RGBA or vis-versa is slow on mac. On my early 2013 MBP, we see a 40% regression when using
the discrete GPU or a 13% regression on integrated.

This change adds a workaround which causes Mac to use the existing Mesa path and prefer readback from the same format as the
framebuffer.

BUG=581311
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Review URL: https://codereview.chromium.org/1786813002

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