frameworks/base: Add support for fully transparent layers

Allow apps to specify a window to be fully transparent. This hint
can be used later by the compositor to drop the layer.

Change-Id: Ib58f1026e74ca39e7160d2adcb34ebf50ec28a9a
Optimize drawBitmapMesh using TRIANGLE_STRIP

Draw BitmapMesh using STRIP instead of TRIANGLES.
Depending on where the bottleneck is located in the pipeline,
there is a potential for saving two-thirds of the time spent rendering.
The speedup is due to avoiding redundant operations,
i.e., mapping each vertex four times to texture and color attribute,
sending each vertex twice to the GPU, vertex processing of GPU ...

In this patch:
1. Refactoring mesh using Strip.
2. Batch triangles by adding degenerate triangles.
3. Optimize computing positions of texture and color attribute.

Benchmark: Drawing Bitmap(1152x768 px) with 200x200 meshes
Performance is doubled (from 26.4 fps to 53 fps, Nexus10).
Memory footprint: only 17% of original method.
(from 240000 to 40401 vertices with xy, uv, rgba data.)

Change-Id: I08c5c20d981d88528ce7b02012feefae468d6ec4
Signed-off-by: kui.zheng <kui.zheng@arm.com>