WIP: implement DMABuf support

[ABeltramo]

See below for more info, here's a TL;DR

Fixes #3

[ABeltramo]

Status update: we can finally exchange DMABuf caps with other plugins and render the wayland output into a DMA buffer to achieve a zero copy pipeline. 🚀

Tested and working pipelines

Simple display output

waylanddisplaysrc ! 'video/x-raw(memory:DMABuf),width=1280,height=720,framerate=60/1' 
! glupload ! glimagesink

Again, just output the buffer

waylanddisplaysrc ! 'video/x-raw(memory:DMABuf),width=1280,height=720,framerate=60/1' 
! glupload ! waylandsink

Nvidia HEVC encoding, decoding and displaying.

waylanddisplaysrc ! 'video/x-raw(memory:DMABuf),width=1280,height=720,framerate=60/1' 
! glupload ! glcolorconvert ! 'video/x-raw(memory:GLMemory),format=NV12' 
!  nvh265enc ! nvh265dec 
! autovideosink

On Nvidia I had to add GST_GL_API=opengl3 to avoid the warning OpenGL context could not support PBO download