gl: implement GstGLMemoryEGL

gl: implement GstGLMemoryEGL

Because current GstEGLImageMemory does not inherit GstGLMemory, GLUpload
allocates additional GLMemory and upload the decoded contents from the decoder
which uses EGLImage (e.g. gst-omx in RPi).

This work adds GstGLMemoryEGL to avoid this overhead. Decoders allocate
GstGLMemoryEGL and decode its contents to the EGLImage of GstGLMemoryEGL.  And
GLUpload uses this memory without allocation of additional textures and blit
operations.

[Matthew Waters]: gst-indent the sources and fix a critical retreiving the egl
display from the memory.

https://bugzilla.gnome.org/show_bug.cgi?id=760916