GtkGLArea had numerous downsides that forced us to invent unsightly hacks
in our renderer to work around them, most chiefly the fact that it holds
its own GdkGLContext on the main thread (GL contexts are not at all
thread-safe), forcing us to keep our GL calls on the main thread.
It also does not interact well with triple-buffering and initialization
is forced to be this sort of deferred song-and-dance since we need to
wait for the GLArea to initialize its GL context before we can initialize
the renderer, the core surface, and then most things in the GTK surface.
We instead invent our own custom widget named RenderSurface that takes
simple DMABUFs and displays them. The task of obtaining a GL context
falls to manual EGL bindings, since we also need EGL to export OpenGL
textures into DMABUFs. We keep the EGL context solely on the render
thread meaning that the main thread never concerns itself with rendering
except when being notified that the renderer has pushed a new frame.
What makes this extra significant is that now the entire GTK apprt no
longer depends on OpenGL in any way, shape or form. As long as it is
being fed DMABUFs, it can render from whichever graphics API you want.
This means we can add more backends based on OpenGL ES or more likely
Vulkan rather painlessly in the future.
**AI disclosure**: I came up with the idea and let Pi implement most of
the nitty-gritty details around EGL, as well as replumbing the renderer
and cleaning up all the GTK-specific workarounds there. I then carefully
vetted every line of code and spent roughly as much time reviewing as
coding. Most of the documentation and all commit messages are in my
own words.
