This change adds a 'SRGB' config flag when creating a SwapChain that
enables linear to sRGB conversion on write.
When using this flag, the linear->srgb conversion in the color grading
post processing should be disabled (or, the whole post-processing
stage should be disabled).
There is also a new query to determine if this flag is supported by
the underlaying platform.
On Metal, this happens automatically when the underlaying layer is sRGB.
This reverts commit 3799e219fc.
5754b9907d