The limitation of four glyph caches per font engine was to prevent memory
spikes during rotation of text. But in an application with multiple top
level OpenGL windows, each with their own context, we'd end up trashing
the list of glyph caches during rendering, even if each window just drew
the same static text.
Having a shared context between the windows helped a bit, but had other
performance issues due to the globally shared state, so the better approach
is to limit the caches to four per context.
This brings the multiwindow manual test from a grinding 4fps on macOS
to a smooth 60fps for 20 concurrent windows.
Task-number: QTBUG-52372
Change-Id: I26edd5f6edb5c7818e14b2203af062df19ae7127
Reviewed-by: Simon Hausmann <simon.hausmann@qt.io>
d856d0e146