Add GLBuffer implementation w/ persistent mapping

If glBufferStorage() is available, we can replace our usage of glBufferSubData() with persistently mapped storage via glMappedBufferRange().

This has 1 disadvantage:

1. It's not supported everywhere, it requires GL 4.4 or GL_EXT_buffer_storage. But every GPU of the last 10 years should implement it. So we check for it and keep the old code. The old code can also be forced via GDK_GL_DISABLE=buffer-storage.

But it has 2 advantages:

1. It is what Vulkan does, so it unifies the two renderers' buffer handling.

2. It is a significant performance boost in use cases with large vertex buffers. Those are pretty rare, but do happen with lots of text at a small font size. An example would be a small font in a maximized VTE terminal or the overview in gnome-text-editor.

A custom benchmark tailored for this problem can be created with:

tests/rendernode-create-tests 1000000 text.node

This creates a node file called "text.node" that draws 1 million text nodes.
(Creating that test takes a minute or so. A smaller number may be useful on less powerful hardware than my Intel Tigerlake laptop.)
The difference can then be compared via:

tools/gtk4-rendernode-tool benchmark --runs=20 text.node

and

GDK_GL_DISABLE=buffer-storage tools/gtk4-rendernode-tool benchmark --runs=20 text.node

Here's a few benchmark numbers from my machines:

computer	size	GL before	GL after	Vulkan before	Vulkan after
TigerLake	1M	1.1s	0.8s	1.0s	1.0s
Radeon RX6550XT	1M	1.6s	0.7s	2.5s	0.9s
Radeon RX6950XT	1M	0.36s	0.3s	1.55s	0.6s
Radeon integrated	1M	1.7s	1.2s	2.8s	1.1s
RPi 4	100k	2.0s	1.9s

And here's the difference in a flamegraph (top is after, bottom is before):

Related: !7021 (closed)