Shadow drawing attempt 3

Fusing the shadow drawing in the shader reduces overdraw which results in faster drawing with less
code. This also fixes a bug with protected resource usage.

Bug: 442023806
Test: Enable com.android.wm.shell.enable_pip_box_shadows and observe the shadows
Flag: com.android.wm.shell.enable_pip_box_shadows
Change-Id: I3485119f9fcc7a7672c08209d9336e75e03764cd