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

For my laptop, the difference is:
before: 1.1s
after:  0.8s

Related: !7021