libgui: add a helper for GLConsumer::getCurrentCrop

Add static GLConsumer::scaleDownCrop for use with
NATIVE_WINDOW_SCALING_MODE_SCALE_CROP.  This also allows it to be
called from other consumers.

Test: libgui_test
Change-Id: I2b5f7d14038b36709b52991b7b7e9379117b708b
2 files changed