ST7789/PicoGraphics: Faster RGB332 mode lookups.

Replace palette search in RGB332 mode with a simple return of the RGB565 colour packed to RGB332,
since this is identical to its index in the palette.

libraries/pico_graphics/pico_graphics.cpp

@@ -25,14 +25,18 @@ namespace pimoroni {
   }
 
   int PicoGraphics::create_pen(uint8_t r, uint8_t g, uint8_t b) {
-    RGB565 c = palette_mode == PaletteModeRGB332 ? create_pen_rgb332(r, g, b) : create_pen_rgb565(r, g, b);
-    int result = search_palette(c);
+    if (palette_mode == PaletteModeRGB332) {
+      return rgb_to_rgb332_index(r, g, b); // Fast pack RGB into palette index
+    } else {
+      RGB565 c = create_pen_rgb565(r, g, b);
+      int result = search_palette(c);
 
-    if (result == -1 && palette_mode == PaletteModeUSER) {
-      result = put_palette(create_pen_rgb565(r, g, b));
+      if (result == -1) {
+        result = put_palette(create_pen_rgb565(r, g, b));
+      }
+
+      return result;
     }
-  
-    return result;
   }
 
   void PicoGraphics::set_pen(uint8_t r, uint8_t g, uint8_t b) {
@@ -48,6 +52,8 @@ namespace pimoroni {
   }
 
   int PicoGraphics::put_palette(RGB565 c) {
+    if(palette_mode != PaletteModeUSER) return -1;
+
     for(auto i = 0u; i < 256; i++) {
       if(!(palette_status[i] & (PaletteStatusUsed | PaletteStatusReserved))) {
         palette[i] = c;
@@ -59,11 +65,15 @@ namespace pimoroni {
   }
 
   void PicoGraphics::set_palette(uint8_t i, RGB565 c) {
+    if(palette_mode != PaletteModeUSER) return;
+
     palette[i] = c;
     palette_status[i] |= PaletteStatusUsed;
   }
 
   int PicoGraphics::reserve_palette() {
+    if(palette_mode != PaletteModeUSER) return - 1;
+
     for (auto i = 0u; i < 256; i++) {
       if (!palette_status[i]) {
         palette_status[i] = PaletteStatusReserved;

libraries/pico_graphics/pico_graphics.hpp

@@ -79,6 +79,10 @@ namespace pimoroni {
     [[deprecated("Use uint8_t create_pen(uint8_t, uint8_t, uint8_t).")]]
     void set_pen(uint8_t r, uint8_t g, uint8_t b);
 
+    static constexpr Pen rgb_to_rgb332_index(uint8_t r, uint8_t g, uint8_t b) {
+      return (r & 0b11100000) | ((g & 0b11100000) >> 3) | ((b & 0b11000000) >> 6);
+    }
+
     static constexpr RGB565 create_pen_rgb565(uint8_t r, uint8_t g, uint8_t b) {
       uint16_t p = ((r & 0b11111000) << 8) |
                    ((g & 0b11111100) << 3) |