output-layer: add cropping support

Add a src_box state field. Use the SRC_* KMS props in the DRM
backend, reject the layers in the Wayland backend (for now, we can
support it later via viewporter).

backend/drm/libliftoff.c

@@ -201,10 +201,18 @@ static bool set_layer_props(struct wlr_drm_backend *drm,
 	uint64_t crtc_w = (uint64_t)state->dst_box.width;
 	uint64_t crtc_h = (uint64_t)state->dst_box.height;
 
-	uint64_t src_x = to_fp16(0);
-	uint64_t src_y = to_fp16(0);
-	uint64_t src_w = to_fp16(width);
-	uint64_t src_h = to_fp16(height);
+	struct wlr_fbox src_box = state->src_box;
+	if (wlr_fbox_empty(&src_box)) {
+		src_box = (struct wlr_fbox){
+			.width = width,
+			.height = height,
+		};
+	}
+
+	uint64_t src_x = to_fp16(src_box.x);
+	uint64_t src_y = to_fp16(src_box.y);
+	uint64_t src_w = to_fp16(src_box.width);
+	uint64_t src_h = to_fp16(src_box.height);
 
 	return
 		liftoff_layer_set_property(layer->liftoff, "zpos", zpos) == 0 &&

backend/wayland/output.c

@@ -294,6 +294,13 @@ static bool output_test(struct wlr_output *wlr_output,
 						height != layer_state->dst_box.height) {
 					supported = false;
 				}
+				if (!wlr_fbox_empty(&layer_state->src_box)) {
+					supported = supported &&
+						layer_state->src_box.x == 0 &&
+						layer_state->src_box.y == 0 &&
+						layer_state->src_box.width == width &&
+						layer_state->src_box.height == height;
+				}
 				supported = supported &&
 					test_buffer(output->backend, layer_state->buffer);
 			}

include/wlr/types/wlr_output_layer.h

@@ -52,6 +52,7 @@ struct wlr_output_layer {
 
 	// private state
 
+	struct wlr_fbox src_box;
 	struct wlr_box dst_box;
 };
 
@@ -63,6 +64,8 @@ struct wlr_output_layer_state {
 
 	// Buffer to display, or NULL to disable the layer
 	struct wlr_buffer *buffer;
+	// Source box, leave empty to use the whole buffer
+	struct wlr_fbox src_box;
 	// Destination box in output-buffer-local coordinates
 	struct wlr_box dst_box;
 

types/output/output.c

@@ -832,6 +832,7 @@ bool wlr_output_commit_state(struct wlr_output *output,
 			wl_list_insert(output->layers.prev, &layer->link);
 
 			// Commit layer state
+			layer->src_box = layer_state->src_box;
 			layer->dst_box = layer_state->dst_box;
 		}
 	}