Optimize shadows on mali, this results in more than a 2x speedup on mali for large windows

On tangor, SF gpu rendering went from 10ms to 4.7ms

Bug: 367464660
Test: Enable enable_freeform_box_shadows from com.android.window.flags. Take a perfetto gpu trace go/perfetto-gpu. The fragment stage after should be shorter.
Flag: EXEMPT bugfix
Change-Id: I68a801e7e292728ce61e71006df6c76bd695ec50