SF: Fix invalid reduction of transparent region from a layer.

Reduce transparent region from a layer only if the transparent
region has valid intersection with the layer. Otherwise, reducing
a transparent region lying completely outside of the layer leads to
invalid sourcecrop values for the layer.

Change-Id: Iee9ba53a8072c21c9ba8a6d58cb14ca30f8600ba
CRs-Fixed: 620973
sf: Determine if virtual display can use HWC for composition

For HWC virtual display using V4L2 approach:
1. HWC virtual display implementation doesn't send output buffer
   back to Surface Flinger.
2. For non-wfd virtual displays(e.g. Simulated Secondary Displays,
   Screen Record, CTS etc..), configure VDS to use GPU for
   composition on output/sink buffer.
3. For wfd virtual display (a.k.a hybrid wfd), use
   Framebuffer surface (similar to external display mechanism).

For HWC virtual display using VDS approach:
1. HWC is allocated for first virtual display connection.
2. Subsequent virtual display sessions will be composed by GLES driver.
ToDo: Modify VDS component to allocate hwcDisplayId based on
mForceHwcCopy (which is based on Usage Flags)

Change-Id: Ib7d2c00ca5fa07908b2eac8085654f8d2cd9af0b