render: optionally clear buffer before rendering

It's suboptimal to do it separately, and some drivers can do it for free this way
Prevents UNASSIGNED-BestPractices-vkCmdClearAttachments-clear-after-load warning

include/wlr/render/pass.h

@@ -27,9 +27,23 @@ struct wlr_render_pass;
  */
 struct wlr_render_timer;
 
+/**
+ * A color value.
+ *
+ * Each channel has values between 0 and 1 inclusive. The R, G, B
+ * channels need to be pre-multiplied by A.
+ */
+struct wlr_render_color {
+	float r, g, b, a;
+};
+
 struct wlr_buffer_pass_options {
 	/* Timer to measure the duration of the render pass */
 	struct wlr_render_timer *timer;
+	/* Clear the buffer before rendering into it */
+	bool clear_buffer;
+	/* If clearing the buffer, what color to clear it with */
+	struct wlr_render_color clear_color;
 };
 
 /**
@@ -93,16 +107,6 @@ struct wlr_render_texture_options {
 void wlr_render_pass_add_texture(struct wlr_render_pass *render_pass,
 	const struct wlr_render_texture_options *options);
 
-/**
- * A color value.
- *
- * Each channel has values between 0 and 1 inclusive. The R, G, B
- * channels need to be pre-multiplied by A.
- */
-struct wlr_render_color {
-	float r, g, b, a;
-};
-
 struct wlr_render_rect_options {
 	/* Rectangle coordinates */
 	struct wlr_box box;

render/gles2/renderer.c

@@ -268,6 +268,14 @@ static struct wlr_render_pass *gles2_begin_buffer_pass(struct wlr_renderer *wlr_
 	if (!pass) {
 		return NULL;
 	}
+	// TODO: switch to using `glClearColor` + `glClear`
+	if (options->clear_buffer) {
+		wlr_render_pass_add_rect(&pass->base, &(struct wlr_render_rect_options){
+			.box = { .width = buffer->buffer->width, .height = buffer->buffer->height },
+			.color = options->clear_color,
+			.blend_mode = WLR_RENDER_BLEND_MODE_NONE,
+		});
+	}
 	return &pass->base;
 }
 

render/pixman/renderer.c

@@ -307,6 +307,13 @@ static struct wlr_render_pass *pixman_begin_buffer_pass(struct wlr_renderer *wlr
 	if (pass == NULL) {
 		return NULL;
 	}
+	if (options->clear_buffer) {
+		wlr_render_pass_add_rect(&pass->base, &(struct wlr_render_rect_options){
+			.box = { .width = buffer->buffer->width, .height = buffer->buffer->height },
+			.color = options->clear_color,
+			.blend_mode = WLR_RENDER_BLEND_MODE_NONE,
+		});
+	}
 	return &pass->base;
 }
 

render/vulkan/renderer.c

@@ -1321,6 +1321,14 @@ static struct wlr_render_pass *vulkan_begin_buffer_pass(struct wlr_renderer *wlr
 	if (render_pass == NULL) {
 		return NULL;
 	}
+	// TODO: switch to using `VkRenderPassBeginInfo.pClearValues`
+	if (options->clear_buffer) {
+		wlr_render_pass_add_rect(&render_pass->base, &(struct wlr_render_rect_options){
+			.box = { .width = buffer->width, .height = buffer->height },
+			.color = options->clear_color,
+			.blend_mode = WLR_RENDER_BLEND_MODE_NONE,
+		});
+	}
 	return &render_pass->base;
 }