Add BGR555 format

The existing formats are

bytes                                  n-bit value
RRRRRRRR GGGGGGGG BBBBBBBB                     BBBBBBBBGGGGGGGGRRRRRRRR
RRRRRRRR GGGGGGGG BBBBBBBB AAAAAAAA    AAAAAAAABBBBBBBBGGGGGGGGRRRRRRRR
GGGRRRRR BBBBBGGG                              BBBBB   GGGGGG  RRRRR

This is
GGGBBBBB xRRRRRGG                      x       RRRRR   GGGGG   BBBBB

So as it's the other way around, call it BGR.

(Even though our RGB565 is a bit backwards, though if you consider them
as packed formats, they're ALL backwards)

32blit/graphics/surface.cpp

@@ -82,6 +82,11 @@ namespace blit {
       bbf = RGBA_RGB565;
       pgf = get_pen_rgb565;
     }break;
+    default: {
+      pbf = nullptr;
+      bbf = nullptr;
+      pgf = nullptr;
+    }break;
     }
   }
 

32blit/graphics/surface.hpp

@@ -80,14 +80,16 @@ namespace blit {
 
     // supported as the screen format
     RGB565 = 4, // red (5-bits), green (6-bits), blue (5bits)
+    BGR555 = 5, // blue (5-bits), green (5-bits), red (5bits)
   };
 
   static const uint8_t pixel_format_stride[] = {
     3,             // RGB
     4,             // RGBA
     1,             // P
     1,             // M
-    2              // RGB565
+    2,             // RGB565
+    2,             // BGR555
   };
 
 #pragma pack(push, 1)