Bug 1669494 - Avoid using glCopyImageSubData on Mali GPUs. r=gw
Bug 1669494 - Avoid using glCopyImageSubData on Mali GPUs. r=gw

On a Mali-T830 it has been observed that glCopyImageSubData calls,
while resizing the webrender picture cache, can take many tens of
milliseconds to complete. They are likely stalling until the previous
frame's rendering to the picture cache has completed. This results in
stuttery scrolling. Using the glBlitFramebuffer path does not
encounter this issue, so use that instead.

As a precaution, apply this workaround to all Mali devices, rather
than just Mali-T.

Differential Revision: https://phabricator.services.mozilla.com/D92638

[ghsync] From https://hg.mozilla.org/mozilla-central/rev/974de9f821d373a2f004a1a0f998244096805006

servo/webrender,servo/webrender,servo/webrender,servo/webrender,servo/webrender,servo/webrender