Add a property to disable libhwui's scissor optimization

Bug #7158326

When scissor optimization is enabled, OpenGLRenderer will attempt to
minimize the use of scissor by selectively enabling and disabling the
GL scissor test.
When the optimization is disabled, OpenGLRenderer will keep the GL
scissor test enabled and change the scissor rect as needed.
Some GPUs (for instance the SGX 540) perform better when changing
the scissor rect often than when enabling/disabling the scissor test
often.

Change-Id: Idb68862e287a23358f9188d577ae0f86161902fd