buffer: make enum wlr_buffer_cap public

Custom backends and renderers need to implement
wlr_backend_impl.get_buffer_caps and
wlr_renderer_impl.get_render_buffer_caps. They can't if enum
wlr_buffer_cap isn't made public.

(cherry picked from commit a38baec1f8)

include/types/wlr_buffer.h

@@ -25,18 +25,6 @@ struct wlr_shm_client_buffer {
 struct wlr_shm_client_buffer *shm_client_buffer_create(
 	struct wl_resource *resource);
 
-/**
- * Buffer capabilities.
- *
- * These bits indicate the features supported by a wlr_buffer. There is one bit
- * per function in wlr_buffer_impl.
- */
-enum wlr_buffer_cap {
-	WLR_BUFFER_CAP_DATA_PTR = 1 << 0,
-	WLR_BUFFER_CAP_DMABUF = 1 << 1,
-	WLR_BUFFER_CAP_SHM = 1 << 2,
-};
-
 /**
  * Get a pointer to a region of memory referring to the buffer's underlying
  * storage. The format and stride can be used to interpret the memory region