Now that LibIPC uses Mach ports for transport on macOS, IOSurface port
rights can be sent as regular IPC message attachments instead of through
a separate ad-hoc Mach message side-channel. Introduce
Web::SharedBackingStore that wraps either a MachPort (macOS) or
ShareableBitmap (other platforms) with IPC encode/decode support,
unifying backing store allocation into the existing
did_allocate_backing_stores IPC message.
Previously, the IOSurface + Mach port path was only taken when a Skia
GPU backend context was available, falling through to the shared memory
path otherwise. This split didn't make sense because IOSurface API
availability doesn't depend on whether GPU painting is enabled.
This change prepares for a future where the rendering thread handles
input events directly, allowing it to trigger repainting without
waiting for the main thread. To support this, the compositor needs to
own the display list, scroll state, and backing stores rather than
receiving them per-frame from the main thread.
To consistently update the backing stores in reallocate_backing_stores()
the variables storing the backing stores need to be set to nullptr at
the start so that they are properly update later on near the end of the
function.
Recently, we moved the backing store manager into Navigable, which means
we now try to allocate a backing store for all navigables, including
those corresponding to SVG image documents. This change disables that
behavior for all navigables except top-level non-SVG traversables,
because otherwise it causes issues when we stop repainting: the browser
process was notified about an allocated backing stores that does not
correspond to the page, and then all subsequent repaints are ignored
until the window is resized.
Now, when Skia backend context is available by the time backing stores
are allocated, there is no need to have a separate BackingStore class.
This allows us to get rid of BackingStore -> PaintingSurface cache.
Making navigables responsible for backing store allocation will allow us
to have separate backing stores for iframes and run paint updates for
them independently, which is a step toward isolating them into separate
processes.
Another nice side effect is that now Skia backend context is ready by
the time backing stores are allocated, so we will be able to get rid of
BackingStore class in the upcoming changes and allocate PaintingSurface
directly.
