Only rescale background images once

This greatly improves performance as rescaling it for every frame is
very expensive.

Resolves #59.

background-image.c

@@ -367,12 +367,13 @@ cairo_surface_t *load_background_image(const char *path) {
 	return image;
 }
 
-void render_background_image(cairo_t *cairo, cairo_surface_t *image,
-		enum background_mode mode, int buffer_width, int buffer_height, double alpha) {
+cairo_surface_t *scale_background_image(cairo_surface_t *image,
+		enum background_mode mode, int buffer_width, int buffer_height) {
+	cairo_surface_t *target = cairo_image_surface_create(CAIRO_FORMAT_RGB24, buffer_width, buffer_height);
+	cairo_t *cairo = cairo_create(target);
 	double width = cairo_image_surface_get_width(image);
 	double height = cairo_image_surface_get_height(image);
 
-	cairo_save(cairo);
 	switch (mode) {
 	case BACKGROUND_MODE_STRETCH:
 		cairo_scale(cairo,
@@ -435,7 +436,16 @@ void render_background_image(cairo_t *cairo, cairo_surface_t *image,
 		assert(0);
 		break;
 	}
+
 	cairo_pattern_set_filter(cairo_get_source(cairo), CAIRO_FILTER_BILINEAR);
+	cairo_paint(cairo);
+	cairo_destroy(cairo);
+	return target;
+}
+
+void render_background_image(cairo_t *cairo, cairo_surface_t *image, double alpha) {
+	cairo_save(cairo);
+	cairo_set_source_surface(cairo, image, 0, 0);
 	cairo_paint_with_alpha(cairo, alpha);
 	cairo_restore(cairo);
 }

include/background-image.h

@@ -19,7 +19,8 @@ enum background_mode parse_background_mode(const char *mode);
 cairo_surface_t *load_background_image(const char *path);
 cairo_surface_t *load_background_from_buffer(void *buf, uint32_t format,
 		uint32_t width, uint32_t height, uint32_t stride, enum wl_output_transform transform);
-void render_background_image(cairo_t *cairo, cairo_surface_t *image,
-		enum background_mode mode, int buffer_width, int buffer_height, double alpha);
+cairo_surface_t *scale_background_image(cairo_surface_t *image,
+		enum background_mode mode, int buffer_width, int buffer_height);
+void render_background_image(cairo_t *cairo, cairo_surface_t *image, double alpha);
 
 #endif

include/swaylock.h

@@ -123,11 +123,13 @@ struct swaylock_state {
 
 struct swaylock_surface {
 	cairo_surface_t *image;
+	cairo_surface_t *scaled_image;
 	struct {
 		uint32_t format, width, height, stride;
 		enum wl_output_transform transform;
 		void *data;
 		cairo_surface_t *original_image;
+		cairo_surface_t *scaled_image;
 		struct swaylock_image *image;
 	} screencopy;
 	struct swaylock_state *state;

render.c

@@ -95,13 +95,25 @@ void render_frame_background(struct swaylock_surface *surface, bool commit) {
 	if (surface->image && state->args.mode != BACKGROUND_MODE_SOLID_COLOR) {
 		cairo_set_operator(cairo, CAIRO_OPERATOR_OVER);
 		if (fade_is_complete(&surface->fade)) {
-			render_background_image(cairo, surface->image,
-				state->args.mode, buffer_width, buffer_height, 1);
+			if (!surface->scaled_image) {
+				surface->scaled_image =
+					scale_background_image(surface->image, state->args.mode,
+						buffer_width, buffer_height);
+			}
+			render_background_image(cairo, surface->scaled_image, 1);
 		} else {
-			render_background_image(cairo, surface->screencopy.original_image,
-				state->args.mode, buffer_width, buffer_height, 1);
-			render_background_image(cairo, surface->image,
-				state->args.mode, buffer_width, buffer_height, surface->fade.alpha);
+			if (!surface->screencopy.scaled_image) {
+				surface->screencopy.scaled_image =
+					scale_background_image(surface->screencopy.original_image,
+						 state->args.mode, buffer_width, buffer_height);
+			}
+			render_background_image(cairo, surface->screencopy.scaled_image, 1);
+			if (!surface->scaled_image) {
+				surface->scaled_image =
+					scale_background_image(surface->image, state->args.mode,
+						buffer_width, buffer_height);
+			}
+			render_background_image(cairo, surface->scaled_image, surface->fade.alpha);
 		}
 	}
 	cairo_restore(cairo);