fix(gx): members of C4Pixel should be unsigned

src/gx/Types.hpp

@@ -338,10 +338,10 @@ enum PIXEL_FORMAT {
 };
 
 struct C4Pixel {
-    char b;
-    char g;
-    char r;
-    char a;
+    uint8_t b;
+    uint8_t g;
+    uint8_t r;
+    uint8_t a;
 };
 
 struct MipBits {