Workaround a NVIDIA/Qt but with black screen in assistants

It seems like Qt uses some internal caches/textures for painting
pixmaps/images on screen. And if the images of one rendering cycle
don't fit into that cache they are painted and black rectangles.

There is a workaround for that: just make the size of the pixmap cache
more than 20 MiB. Then all the pixmaps painted through the cache will work
correctly (if you decide to draw a QImage on screen manually, it still
doesn't work).

I don't know what happens there, but it seems like this workaround fixes
the problem. Let's wait until we merge Qt+openGL3 branch, probably, it
will change something.

BUG:361709