grutil: Add clear_color method to CardMaker

panda/src/grutil/cardMaker.I

@@ -101,6 +101,15 @@ set_color(const LVecBase4 &color) {
   _has_color = true;
 }
 
+/**
+ * Unsets the color of the card.
+ */
+INLINE void CardMaker::
+clear_color() {
+  _has_color = false;
+  _color.set(1.0f, 1.0f, 1.0f, 1.0f);
+}
+
 /**
  * Sets the flag indicating whether vertices will be generated with normals or
  * not.  Normals are required if you intend to enable lighting on the card,

panda/src/grutil/cardMaker.h

@@ -49,6 +49,7 @@ PUBLISHED:
 
   INLINE void set_color(PN_stdfloat r, PN_stdfloat g, PN_stdfloat b, PN_stdfloat a);
   INLINE void set_color(const LColor &color);
+  INLINE void clear_color();
 
   INLINE void set_has_normals(bool flag);