linux_dmabuf_v1: Implement data_ptr_access

Allow direct access to the pixel data of linux_dmabuf_v1 buffers by
mmapping the FD.  This causes a wait on any outstanding fences and also
triggers the DMA_BUF_SYNC mechanism to do any cache fiddling needed.

This doesn't support multi-planar formats (e.g. YUV420 from hardware
codecs)

I also fix the comment on wlr_renderer.render_buffer_caps (it's used for
textures, not the render target) and update
wlr_renderer_init_wl_display() and
wlr_linux_dmabuf_feedback_v1_init_with_options() to use the renderer's
own claimed buffer_caps instead of hardcoding DMABUF as required.

Loosely based on 46ef2cfa3c

include/wlr/render/wlr_renderer.h

@@ -26,8 +26,8 @@ struct wlr_fbox;
  * A renderer for basic 2D operations.
  */
 struct wlr_renderer {
-	// Capabilities required for the buffer used as a render target (bitmask of
-	// enum wlr_buffer_cap)
+	// Capabilities required for the buffers used as texture sources and
+	// render target (bitmask of enum wlr_buffer_cap)
 	uint32_t render_buffer_caps;
 
 	struct {