963559 - Use deferred canvas in DrawTargetSkia

Status: NEW

(Core :: Graphics: Canvas2D, defect)

Description

[James Willcox (:snorp) ([e-mail]) (he/him)]

Using SkDeferredCanvas or similar should allows to make several optimizations that are impossible right now in the context of SkiaGL. Most importantly:

1) Avoid creating a GL canvas if there is some operation that makes that a bad idea. Bug 963268 is a good example of this. The snapshot code does:

    context.scale(scale, scale);
    context.drawWindow(window, 0, 0, width, height,
                       'rgb(255,255,255)', flags);

We want drawWindow() types of usage to have a software canvas, but the scale() call forces a DrawTarget to be created, which will be GL by default. If we have the deferred canvas, we can wait until everything is done to flush it to whatever type of DrawTarget we want.

2) Right now with SkiaGL we always make a copy of the current back buffer after a "frame" has been drawn. With the deferred canvas we can (more easily) detect when the entire canvas is dirty and just skip that step. This should make frame-based games and other similar usages a little faster.