ASTC normal map support

- Update astc to 57af6af from https://github.com/andrewwillmott/astc-encoder
- Expand bimg::Quality to include source-as-normal-map
  modes, intended as a hint to the encoder.
- Hook this up for ASTC, and in texturec

3rdparty/astc/mathlib.cpp

@@ -61,7 +61,7 @@ float nan(int p)
 	return v.q;
 }
 
-#if !defined(_MSC_VER) && (__cplusplus < 201103L)
+#if !_MSC_VER && !__clang__ && (__cplusplus < 201103L)
 float fmax(float p, float q)
 {
 	if (p != p)