We want to allow sRGB conversion always, except when blitting to/from
a buffer. This mimics how it's done in the non-VK path.

I also opportunistically made Options constructors explicit to make it
easier to locate instantiation sites, and renamed convertSRGB to
allowSRGBConversion to make it clear that setting this to true doesn't
mean SRGB conversion will necessarily happen.

Bug: angleproject:4011
Change-Id: I88c229e683d26246b21ef601e066df6eb429e0a6
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/37468
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Tested-by: Antonio Maiorano <amaiorano@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>