[API] Add gstgralloc library

Submitted by Mohammed Sameer

Link to original bug (#734908)

Description

Created attachment 283596
proposed patch

Attaching a patch that adds a GstGralloc allocator with its GstGrallocMemory and also a buffer pool (GstGrallocBufferPool) for usage with elements utilizing the above mentioned allocator.

gralloc is the library used by Android to allocate android native buffers used for graphics rendering and zero copy media decoding (Probably it has other uses as well).

The only issue I see with the patch is I had to lie and set the format to GST_VIDEO_FORMAT_I420 because I cannot use GST_VIDEO_FORMAT_ENCODED since it's size is 0 and various GStreamer API dealing with allocation queries expect size which is not 0

The allocator will be used to add support to gst-omx (And perhaps glimagesink too)

Patch 283596, "proposed patch":
0001-Add-gstgralloc-library.patch