Support source blending to the final framebuffer

[kvark]

https://bugzilla.mozilla.org/show_bug.cgi?id=1391322

See CAIRO_OPERATOR_SOURCE from https://www.cairographics.org/operators/

Not entirely sure this is needed. We may use this issue as a space for discussion and close it afterwards.
Gecko would provide us with a list of rectangles, which we would then need to ensure are not affected by any rendering. This can be done by rendering them with an opposite alpha from what our framebuffer is cleared with, right after the clear, and then compositing the otherwise final picture onto it with blending that excludes those areas.

[glennw]

Could you expand a little bit on the use case for this?

[jrmuizel]

This is used for making parts of the back buffer transparent for the vibrancy effect on OS X and cutting out the spots for window controls on Windows.

[glennw]

@jrmuizel Thanks - does this require anti-aliasing around the edges, or is it a straight in/out mask?

[jrmuizel]

I think pixel aligned rectangles are probably sufficient for our needs.

[glennw]

@jrmuizel OK - I wonder if it makes sense to just use the stencil buffer for this then?

[jrmuizel]

Possibly but that nearly feels like overkill. Let's look more closely at what an appropriate solution will be once we hook up the gecko side.

[glennw]

Sure, sounds good.

[mstange]

This is superseded by #1926 and can be closed.