buffer: reduce unnecessary painting to new cairo surfaces

Add buffer_adopt_cairo_surface(), which allows wrapping an existing
cairo image surface in a struct lab_data_buffer. This is useful when
loading PNGs since most will be loaded as ARGB32 already.

Fix a memory leak in the non-ARGB32 PNG case, where we do still need to
paint to a new image surface -- we were leaking the original surface.

Eliminate an unnecessary temporary image surface in SVG loading and just
render the SVG to the image surface held by the lab_data_buffer.

I also cleaned up and clarified the buffer API a bit:

- Add a pointer to the held cairo_surface_t (so we can still access it
  if there is no cairo_t).
- Remove the free_on_destroy bool (it was always true).
- Rename unscaled_width/height to logical_width/height and add an
  explanatory comment. It was unclear what "unscaled" meant.
- Rename buffer_create_wrap() to buffer_create_from_data().

This is laying groundwork for some more icon fixes I am working on
(making sure icons are loaded and rendered at the correct scale).

src/common/font.c

@@ -109,7 +109,7 @@ font_buffer_create(struct lab_data_buffer **buffer, int max_width,
 	}
 
 	*buffer = buffer_create_cairo(text_extents.width + arrow_extents.width,
-			text_extents.height, scale, true);
+			text_extents.height, scale);
 	if (!*buffer) {
 		wlr_log(WLR_ERROR, "Failed to create font buffer");
 		return;

src/common/graphic-helpers.c

@@ -125,8 +125,8 @@ get_cairo_surface_from_lab_data_buffer(struct lab_data_buffer *buffer)
 	}
 
 	/* Handle DRM_FORMAT_ARGB8888 buffers */
-	int w = buffer->unscaled_width;
-	int h = buffer->unscaled_height;
+	int w = buffer->logical_width;
+	int h = buffer->logical_height;
 	cairo_surface_t *surface =
 		cairo_image_surface_create(CAIRO_FORMAT_ARGB32, w, h);
 	if (!surface) {

src/common/scaled-font-buffer.c

@@ -25,8 +25,8 @@ _create_buffer(struct scaled_scene_buffer *scaled_buffer, double scale)
 		wlr_log(WLR_ERROR, "font_buffer_create() failed");
 	}
 
-	self->width = buffer ? buffer->unscaled_width : 0;
-	self->height = buffer ? buffer->unscaled_height : 0;
+	self->width = buffer ? buffer->logical_width : 0;
+	self->height = buffer ? buffer->logical_height : 0;
 	return buffer;
 }
 

src/common/scaled-scene-buffer.c

@@ -72,8 +72,8 @@ _update_buffer(struct scaled_scene_buffer *self, double scale)
 		/* Ensure the buffer doesn't get deleted behind our back */
 		wlr_buffer_lock(&buffer->base);
 	}
-	self->width = buffer ? buffer->unscaled_width : 0;
-	self->height = buffer ? buffer->unscaled_height : 0;
+	self->width = buffer ? buffer->logical_width : 0;
+	self->height = buffer ? buffer->logical_height : 0;
 
 	/* Create or reuse cache entry */
 	if (wl_list_length(&self->cache) < LAB_SCALED_BUFFER_MAX_CACHE) {