EGL/android: pbuffer implementation

EGL/android: pbuffer implementation

Android path didn't support pbuffer, so add pbuffer support to fix
most failing dEQP and CTS pbuffer test cases.

Patch adds a single buffer config to support pbuffer, and creates
image in getBuffers for pbuffer when surface type is front surface.

The EGL 1.5 spec states that pbuffers have a back buffer but no front
buffer, single-buffered surfaces with no front buffer confuse Mesa;
so we deviate from the spec, following the precedent of Mesa's EGL
X11 platform.

V3: update commit message and code review changes.

Signed-off-by: Liu Zhiquan <zhiquan.liu@intel.com>
Signed-off-by: Kalyan Kondapally <kalyan.kondapally@intel.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>