Removing State and Restore reduces painting commands count by 30-50%
on an average website.
Due to this change, it was also necessary to replace AddClipRect with
SetClipRect command.
By storing painting command coordinates relative to the nearest
stacking context we can get rid of the Translate command.
Additionally, this allows us to easily check if the bounding
rectangles of the commands cover or intersect within a stacking
context. This should be useful if we decide to optimize by avoiding
the execution of commands that will be overpainted by the results of
subsequent commands.
This change separates the box outer shadow metrics calculations into a
separate function. This function is then used to obtain the shadow
bounding rectangle and skip painting if the entire shadow is outside
of the viewport.
This modification introduces a new layer to the painting process. The
stacking context traversal no longer immediately calls the
Gfx::Painter methods. Instead, it writes serialized painting commands
into newly introduced RecordingPainter. Created list of commands is
executed later to produce resulting bitmap.
Producing painting command list will make it easier to add new
optimizations:
- It's simpler to check if the painting result is not visible in the
viewport at the command level rather than during stacking context
traversal.
- Run painting in a separate thread. The painting thread can process
serialized painting commands, while the main thread can work on the
next paintable tree and safely invalidate the previous one.
- As we consider GPU-accelerated painting support, it would be easier
to back each painting command rather than constructing an alternative
for the entire Gfx::Painter API.
