Add renderer function to clear rectangle

renderer.c

@@ -125,11 +125,6 @@ void renderer_handle()
 	drmHandleEvent(drm_fd, &ev);
 }
 
-void renderer_clear()
-{
-	memset(fbs[back].buf, 0, fbs[back].size);
-}
-
 void renderer_swap()
 {
 	const int err = drmModePageFlip(
@@ -141,6 +136,29 @@ void renderer_swap()
 	back = (back + 1) & 1;
 }
 
+void renderer_clear()
+{
+	memset(fbs[back].buf, 0, fbs[back].size);
+}
+
+void renderer_clear_rect(vec2_t centre, vec2_t size)
+{
+	const vec2_t tl_world = { centre.x - size.x / 2, centre.y + size.y / 2 };
+	const vec2_t br_world = { centre.x + size.x / 2, centre.y - size.y / 2 };
+	const vec3_t tl = mat3_mul_vec3(view, vec2_extend(tl_world));
+	const vec3_t br = mat3_mul_vec3(view, vec2_extend(br_world));
+
+	const unsigned width = (unsigned)roundf(br.x / br.z - tl.x / tl.z);
+	const unsigned x1 = (unsigned)roundf(tl.x / tl.z);
+	const unsigned y1 = (unsigned)roundf(tl.y / tl.z);
+	const unsigned y2 = (unsigned)roundf(br.y / br.z);
+
+	for (unsigned y = y1; y < y2; ++y) {
+		uint32_t *row = fbs[back].buf + y * (fbs[back].pitch / 4) + x1;
+		memset(row, 0, width * sizeof(uint32_t));
+	}
+}
+
 void renderer_draw(
     const vec2_t *vs, unsigned count, mat3_t model, bool connect)
 {

renderer.h

@@ -16,9 +16,11 @@ renderer_params_t renderer_init();
 void renderer_cleanup();
 
 void renderer_handle();
-void renderer_clear();
 void renderer_swap();
 
+void renderer_clear();
+void renderer_clear_rect(vec2_t centre, vec2_t size);
+
 void renderer_draw(
     const vec2_t *vs, unsigned count, mat3_t model, bool connect);