Found on rpi when gpu_mem is too low so there is not enough memory to
create the eglimage. But still gst_buffer_pool_acquire_buffer succeeded.
And it leads to a CRITICAL assert:
gst_egl_image_get_image: assertion 'GST_IS_EGL_IMAGE (image)' failed
https://bugzilla.gnome.org/show_bug.cgi?id=785518
Rather than assuming something. e.g. zerocopy on iOS with GLES3 requires
the use of Luminance/Luminance Alpha formats and does not work with
Red/RG textures.
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
