vdev/wm: Allow DRM rendering

[obiwac]

Allow rendering to DRM. Previously, the swapchain buffers wouldn't be written to at all, and so they'd just flash between pure black and pure white as their default contents, at least on my machine (AMD 7040, amdgpu, FreeBSD).

Here are the other issues that need to be fixed before we can merge this:

• Currently, we're hardcoding resolution to 1280x720.
• We're breaking UI frost to be able to render to the swapchain buffer, as we can't use it as a copy texture destination as we were doing previously. See the description of 67bc87d.
• We still haven't really gotten to the bottom of what's happening. We've just narrowed it down and have a horrible disgusting hack as a workaround atm.
• Related to previous point, but we still have some crazy visual glitches, probably due to our pretty much non-existent approach to rigorous synchronization.

Here are the notes I had in #33 before opening this PR:

Flashing black and white when rendering directly to DRM (instead of in nested window)

Presumably, one of the swapchain buffers is purely white and the other is purely black, and we're flipping between these. Probably some sync error where we're not waiting for rendering to complete. To figure out where the problem is (bearing in mind I don't understand what I'm doing), I'm trying to disable stuff in the simple example until we get the same behaviour (in render_pass_submit(), where simple is doing its queue submission):

• Checked vulkan_sync_render_buffer().
• Checked pWaitSemaphoreInfos in VkSubmitInfo2KHR. (Would need an API for giving these to the Vulkan wgpu-hal for its submission.)
• Checked pSignalSemaphoreInfos.

Here's a couple assumptions I need to verify:

• Is it really the case that one of the swapchain buffers is pure black and the other white? Can I prove this by just displaying one or the other? Why could this be the case.