buffer: add buffer_convert_cairo_surface_for_icon()

Which handles:
 - conversion from other pixel formats to ARGB32
 - setting the logical size to the desired display size
 - downscaling very large images using CAIRO_FILTER_GOOD

include/buffer.h

@@ -61,6 +61,18 @@ struct lab_data_buffer *buffer_adopt_cairo_surface(cairo_surface_t *surface);
 struct lab_data_buffer *buffer_create_cairo(uint32_t logical_width,
 	uint32_t logical_height, float scale);
 
+/*
+ * Create a buffer from an image surface, for display as an icon.
+ *
+ * The surface is either adopted by the buffer (which takes ownership),
+ * or copied and then destroyed.
+ *
+ * This function allows non-ARGB32 source images and converts to
+ * CAIRO_FORMAT_ARGB32 if needed.
+ */
+struct lab_data_buffer *buffer_convert_cairo_surface_for_icon(
+	cairo_surface_t *surface, uint32_t icon_size, float scale);
+
 /*
  * Create a buffer which holds (and takes ownership of) raw pixel data
  * in pre-multiplied ARGB32 format.

src/buffer.c

@@ -29,6 +29,7 @@
 #include <drm_fourcc.h>
 #include <wlr/interfaces/wlr_buffer.h>
 #include "buffer.h"
+#include "common/box.h"
 #include "common/mem.h"
 
 static const struct wlr_buffer_impl data_buffer_impl;
@@ -134,6 +135,58 @@ buffer_create_cairo(uint32_t logical_width, uint32_t logical_height, float scale
 	return buffer;
 }
 
+struct lab_data_buffer *
+buffer_convert_cairo_surface_for_icon(cairo_surface_t *surface,
+		uint32_t icon_size, float scale)
+{
+	assert(cairo_surface_get_type(surface) == CAIRO_SURFACE_TYPE_IMAGE);
+
+	/*
+	 * Compute logical size for display and decide whether we can
+	 * use the image data directly (fast path). Requirements are:
+	 *
+	 *  - The pixel format must be ARGB32.
+	 *  - The image must not be so large as to need downsampling by
+	 *    more than 2x when displayed at the target scale. wlr_scene
+	 *    uses linear interpolation without pixel averaging, which
+	 *    starts to skip samples if downsampling more than 2x,
+	 *    resulting in a grainy look.
+	 */
+	int width = cairo_image_surface_get_width(surface);
+	int height = cairo_image_surface_get_height(surface);
+	struct wlr_box logical =
+		box_fit_within(width, height, icon_size, icon_size);
+	struct lab_data_buffer *buffer;
+
+	if (cairo_image_surface_get_format(surface) == CAIRO_FORMAT_ARGB32
+			&& width <= 2 * logical.width * scale
+			&& height <= 2 * logical.height * scale) {
+		buffer = buffer_adopt_cairo_surface(surface);
+		/* set logical size for display */
+		buffer->logical_width = logical.width;
+		buffer->logical_height = logical.height;
+	} else {
+		/* convert to ARGB32 and scale for display (slow path) */
+		buffer = buffer_create_cairo(logical.width,
+			logical.height, scale);
+
+		cairo_t *cairo = buffer->cairo;
+		cairo_scale(cairo, (double)logical.width / width,
+			(double)logical.height / height);
+		cairo_set_source_surface(cairo, surface, 0, 0);
+		cairo_pattern_set_filter(cairo_get_source(cairo),
+			CAIRO_FILTER_GOOD);
+		cairo_paint(cairo);
+
+		/* ensure pixel data is updated */
+		cairo_surface_flush(buffer->surface);
+		/* destroy original surface */
+		cairo_surface_destroy(surface);
+	}
+
+	return buffer;
+}
+
 struct lab_data_buffer *
 buffer_create_from_data(void *pixel_data, uint32_t width, uint32_t height,
 		uint32_t stride)