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/img/img-png.c

@@ -64,10 +64,20 @@ img_png_load(const char *filename, struct lab_data_buffer **buffer)
 	}
 	cairo_surface_flush(image);
 
-	double w = cairo_image_surface_get_width(image);
-	double h = cairo_image_surface_get_height(image);
-	*buffer = buffer_create_cairo((int)w, (int)h, 1.0, true);
+	if (cairo_image_surface_get_format(image) == CAIRO_FORMAT_ARGB32) {
+		/* we can wrap ARGB32 image surfaces directly */
+		*buffer = buffer_adopt_cairo_surface(image);
+		return;
+	}
+
+	/* convert to ARGB32 by painting to a new surface */
+	uint32_t w = cairo_image_surface_get_width(image);
+	uint32_t h = cairo_image_surface_get_height(image);
+	*buffer = buffer_create_cairo(w, h, 1);
 	cairo_t *cairo = (*buffer)->cairo;
 	cairo_set_source_surface(cairo, image, 0, 0);
-	cairo_paint_with_alpha(cairo, 1.0);
+	cairo_paint(cairo);
+	cairo_surface_flush((*buffer)->surface);
+	/* destroy original surface */
+	cairo_surface_destroy(image);
 }

src/img/img-svg.c

@@ -39,8 +39,9 @@ img_svg_load(const char *filename, struct lab_data_buffer **buffer,
 		return;
 	}
 
-	cairo_surface_t *image = cairo_image_surface_create(CAIRO_FORMAT_ARGB32, size, size);
-	cairo_t *cr = cairo_create(image);
+	*buffer = buffer_create_cairo(size, size, 1.0);
+	cairo_surface_t *image = (*buffer)->surface;
+	cairo_t *cr = (*buffer)->cairo;
 
 	rsvg_handle_render_document(svg, cr, &viewport, &err);
 	if (err) {
@@ -55,15 +56,11 @@ img_svg_load(const char *filename, struct lab_data_buffer **buffer,
 	}
 	cairo_surface_flush(image);
 
-	double w = cairo_image_surface_get_width(image);
-	double h = cairo_image_surface_get_height(image);
-	*buffer = buffer_create_cairo((int)w, (int)h, 1.0, /* free_on_destroy */ true);
-	cairo_t *cairo = (*buffer)->cairo;
-	cairo_set_source_surface(cairo, image, 0, 0);
-	cairo_paint_with_alpha(cairo, 1.0);
+	g_object_unref(svg);
+	return;
 
 error:
-	cairo_destroy(cr);
-	cairo_surface_destroy(image);
+	wlr_buffer_drop(&(*buffer)->base);
+	*buffer = NULL;
 	g_object_unref(svg);
 }

src/img/img-xbm.c

@@ -266,8 +266,8 @@ img_xbm_from_bitmap(const char *bitmap, struct lab_data_buffer **buffer,
 	}
 	color = argb32(rgba);
 	pixmap = parse_xbm_builtin(bitmap, 6);
-	*buffer = buffer_create_wrap(pixmap.data, pixmap.width, pixmap.height,
-		pixmap.width * 4, /* free_on_destroy */ true);
+	*buffer = buffer_create_from_data(pixmap.data, pixmap.width, pixmap.height,
+		pixmap.width * 4);
 }
 
 void
@@ -298,9 +298,9 @@ img_xbm_load(const char *filename, struct lab_data_buffer **buffer,
 		return;
 	}
 
-	/* Create buffer with free_on_destroy being true */
+	/* Create buffer */
 	if (pixmap.data) {
-		*buffer = buffer_create_wrap(pixmap.data, pixmap.width,
-			pixmap.height, pixmap.width * 4, true);
+		*buffer = buffer_create_from_data(pixmap.data, pixmap.width,
+			pixmap.height, pixmap.width * 4);
 	}
 }

src/img/img-xpm.c

@@ -387,7 +387,7 @@ pixbuf_create_from_xpm(struct file_handle *handle)
 	free(colors);
 	free(name_buf);
 
-	return buffer_create_wrap(data, w, h, 4 * w, true);
+	return buffer_create_from_data(data, w, h, 4 * w);
 
 out:
 	g_hash_table_destroy(color_hash);