LibSoftGPU: Configure stats overlay period

Problem:
- The statistics overlay period is hardcoded to 500 ms. This time is
  very short and can result in the values being very "jumpy".

Solution:
- Increasing this value can result in more steady values which is
  useful when trying to evaluate the performance impact of a change. A
  new config value is offered in `Config.h` to let the developer
  change to any value desired.

Userland/Libraries/LibSoftGPU/Device.cpp

@@ -1224,7 +1224,7 @@ void Device::draw_statistics_overlay(Gfx::Bitmap& target)
 
     Gfx::Painter painter { target };
 
-    if (milliseconds > 500) {
+    if (milliseconds > MILLISECONDS_PER_STATISTICS_PERIOD) {
 
         int num_rendertarget_pixels = m_render_target->width() * m_render_target->height();