For now, OpenGL and Vulkan renderers are broken as I focused on getting
the software renderer working (which was quite tricky to get right).
This fixes a couple of issues: the segfault when warping the screen (due
to the scene rendering move invalidating the warp buffer), and warp
always having 320x200 resolution. There's still the problem of the
effect being too subtle at high resolution, but that's just a matter of
updating the tables and tweaking the code in D_WarpScreen.
Another issue is the Draw functions should probably write directly to
the main frame buffer or even one passed in as a parameter. This would
remove the need for binding the main buffer at the beginning and end of
the frame.
GL still has its own functions for enabling and disabling fog while
rendering, but GLSL doesn't need such (thanks to the shaders), nor will
vulkan (and the software renderers don't support fog).
This is a step towards high-level unification of the renderers, as far
as possible keeping only actual low-level implementation details in the
individual renderers (some higher level stuff, eg shadows, is expected
to be per-renderer as some things are just not feasible to implement in
all renderers). However, the idea is to move the high-level
functionality into scene rendering.
This paves the way for clean initialization of the Vulkan renderer, and
very much cleans up the older renderer initialization code as gl and sw
are no longer intertwined.
