Short circuit the gainmap shader if we would not apply the gainmap

This is needed for correctness when authoring to a PDF. The gainmap
shader otherwise manually de-gammas into a working space to do the
gainmap math, then uses toLinearSrgb to re-gamma to the destination
space.

But, SkPicture records commands while being colorspace agnostic, so
manually de-gammaing means we never re-gamma, which has the effect over
crushing the resulting image.

This is probably also nice for performance too, because we avoid
sampling the gainmap and a bunch of math operations.

Bug: 341006313
Test: Photos > 1up > Print
Flag: EXEMPT low risk bug fix
Change-Id: Id2de97b112a8eff8ef56a802a3f44d83abdd6d41