LibGfx: Improve glyph rendering speed for vector fonts

The glyph bitmap is a grayscale image that is multiplied with the
requested color provided to `Gfx::Painter::draw_glyph()` to get the
final glyph bitmap that can be blitted.

Using `Gfx::Color::multiply()` is unnecessary however: by simply taking
the destination color and copying over the glyph bitmap's alpha value,
we can prevent four multiplications and divisions per pixel.

In an artifical benchmark I wrote, this improved glyph rendering
performance by ~9%.

Userland/Libraries/LibGfx/Painter.cpp

@@ -1432,7 +1432,7 @@ FLATTEN void Painter::draw_glyph(FloatPoint point, u32 code_point, Font const& f
         draw_scaled_bitmap(rect.to_rounded<int>(), *glyph.bitmap(), glyph.bitmap()->rect(), 1.0f, ScalingMode::BilinearBlend);
     } else {
         blit_filtered(glyph_position.blit_position, *glyph.bitmap(), glyph.bitmap()->rect(), [color](Color pixel) -> Color {
-            return pixel.multiply(color);
+            return color.with_alpha(pixel.alpha());
         });
     }
 }