Actually use texture bounds in CachedSet::getTextureBounds

Virtual displays may be projected onto a display area that is a
subrectangle of the display size. Because the flattened buffer for a
CachedSet is display-size, we need to use the display size as the
display frame instead of the framebuffer output rectangle. Otherwise, we
would be blitting the contents of the display-sized texture onto a
smaller area, which causes an unintended scale.

Bug: 191328868
Test: Simulating secondary displays does not squash the content.
Test: libcompositionengine_test
Change-Id: I79d56a998e4f7fc030191fed50002354c3c18561