GUI: prepare for new pattern renderer

likely two stages, depending on how efficient the first one is

stage 1:
- no more Selectables
- using ImDrawList to draw the pattern
  - perhaps even bypassing that and directly firing quads at the draw queue

stage 2:
- using textures and tiles to draw the pattern

src/gui/pattern.cpp

@@ -419,6 +419,11 @@ inline void FurnaceGUI::patternRow(int i, bool isPlaying, float lineHeight, int
 }
 
 void FurnaceGUI::drawPattern() {
+  if (newPatternRenderer) {
+    drawPatternNew();
+    return;
+  }
+
   //int delta0=SDL_GetPerformanceCounter();
   if (nextWindow==GUI_WINDOW_PATTERN) {
     patternOpen=true;