Add support function for stb compressor.

src/nvimage/ColorBlock.cpp

@@ -93,6 +93,15 @@ void ColorBlock::swizzleDXT5n()
 	}
 }
 
+void ColorBlock::swizzleSTB()
+{
+	for(int i = 0; i < 16; i++)
+	{
+		Color32 c = m_color[i];
+		m_color[i] = Color32(c.b, c.g, c.r, c.a);
+	}
+}
+
 void ColorBlock::splatX()
 {
 	for(int i = 0; i < 16; i++)

src/nvimage/ColorBlock.h

@@ -20,6 +20,7 @@ namespace nv
 		void init(const Image * img, uint x, uint y);
 		
 		void swizzleDXT5n();
+		void swizzleSTB();
 		void splatX();
 		void splatY();