This is in preparation of vulkan backend. We move all opengl
functionnalities behind an abstract class.
This also cleansup the "dynamic" ubo create and rename it to
`GPU_uniformbuf_from_list()`
Contains, no functional change.
Part of T68990 Vulkan support.
This just set a global object responsible for allocating new objects
in a thread safe way without needing any GPUContext bound to this
thread.
This also introduce the GLContext which will contain all the GL related
functions for the current context.
