render: introduce wlr_swapchain

The swapchain maximum capacity is set to 4, so that we have enough room
for:

- A buffer currently displayed on screen
- A buffer queued for display (e.g. to KMS)
- A pending buffer that'll be queued next commit
- An additional pending buffer in case we want to invalidate the
  currently pending one

render/meson.build

@@ -8,6 +8,7 @@ wlr_files += files(
 	'gles2/renderer.c',
 	'gles2/shaders.c',
 	'gles2/texture.c',
+	'swapchain.c',
 	'wlr_renderer.c',
 	'wlr_texture.c',
 )