Implement faster version of ClearRect

Is 30% faster than the Pixman version so lets use it everywhere

src/bitmap.cpp

@@ -799,18 +799,12 @@ void Bitmap::Clear() {
 }
 
 void Bitmap::ClearRect(Rect const& dst_rect) {
-	pixman_color_t pcolor = {};
-	pixman_box32_t box = {
-		dst_rect.x,
-		dst_rect.y,
-		dst_rect.x + dst_rect.width,
-		dst_rect.y + dst_rect.height
-	};
-
-	box.x2 = Utils::Clamp<int32_t>(box.x2, 0, width());
-	box.y2 = Utils::Clamp<int32_t>(box.y2, 0, height());
+	if (dst_rect == GetRect()) {
+		Clear();
+		return;
+	}
 
-	pixman_image_fill_boxes(PIXMAN_OP_CLEAR, bitmap.get(), &pcolor, 1, &box);
+	BitmapBlit::ClearRect(*this, dst_rect);
 }
 
 // Hard light lookup table mapping source color to destination color

src/bitmap_blit.cpp

@@ -235,4 +235,20 @@ bool BlitFast(Bitmap& dest, int x, int y, Bitmap const& src, Rect src_rect,
 	return true;
 }
 
+void ClearRect(Bitmap& dest, Rect src_rect) {
+	src_rect.Adjust(dest.GetRect());
+	Rect& dst_rect = src_rect;
+
+	int bpp = dest.bpp();
+	int dst_pitch = dest.pitch();
+
+	uint8_t* dst_pixels = (uint8_t*)dest.pixels() + dst_rect.x * bpp + dst_rect.y * dst_pitch;
+
+	int line_width = src_rect.width * bpp;
+
+	for (int y = 0; y < src_rect.height; ++y) {
+		memset(dst_pixels + y * dst_pitch, '\0', line_width);
+	}
+}
+
 } // namespace BitmapBlit

src/bitmap_blit.h

@@ -27,6 +27,8 @@ namespace BitmapBlit {
 
 	bool BlitFast(Bitmap& dest, int x, int y, Bitmap const& src, Rect src_rect,
 		Opacity const& opacity);
+
+	void ClearRect(Bitmap& dest, Rect src_rect);
 }
 
 #endif