Add etcpak library for faster ETC/ETC2/S3TC imports.

- `etc` module was renamed to `etcpak` and modified to use the new library.
- PKM importer is removed in the process, it's obsolete.
- Old library `etc2comp` is removed.
- S3TC compression no longer done via `squish` (but decompression still is).
- Slight modifications to etcpak sources for MinGW compatibility,
  to fix LLVM `-Wc++11-narrowing` errors, and to allow using vendored or
  system libpng.

Co-authored-by: Rémi Verschelde <rverschelde@gmail.com>

thirdparty/etcpak/Math.hpp

@@ -0,0 +1,92 @@
+#ifndef __DARKRL__MATH_HPP__
+#define __DARKRL__MATH_HPP__
+
+#include <algorithm>
+#include <cmath>
+#include <stdint.h>
+
+#include "ForceInline.hpp"
+
+template<typename T>
+static etcpak_force_inline T AlignPOT( T val )
+{
+    if( val == 0 ) return 1;
+    val--;
+    for( unsigned int i=1; i<sizeof( T ) * 8; i <<= 1 )
+    {
+        val |= val >> i;
+    }
+    return val + 1;
+}
+
+static etcpak_force_inline int CountSetBits( uint32_t val )
+{
+    val -= ( val >> 1 ) & 0x55555555;
+    val = ( ( val >> 2 ) & 0x33333333 ) + ( val & 0x33333333 );
+    val = ( ( val >> 4 ) + val ) & 0x0f0f0f0f;
+    val += val >> 8;
+    val += val >> 16;
+    return val & 0x0000003f;
+}
+
+static etcpak_force_inline int CountLeadingZeros( uint32_t val )
+{
+    val |= val >> 1;
+    val |= val >> 2;
+    val |= val >> 4;
+    val |= val >> 8;
+    val |= val >> 16;
+    return 32 - CountSetBits( val );
+}
+
+static etcpak_force_inline float sRGB2linear( float v )
+{
+    const float a = 0.055f;
+    if( v <= 0.04045f )
+    {
+        return v / 12.92f;
+    }
+    else
+    {
+        return pow( ( v + a ) / ( 1 + a ), 2.4f );
+    }
+}
+
+static etcpak_force_inline float linear2sRGB( float v )
+{
+    const float a = 0.055f;
+    if( v <= 0.0031308f )
+    {
+        return 12.92f * v;
+    }
+    else
+    {
+        return ( 1 + a ) * pow( v, 1/2.4f ) - a;
+    }
+}
+
+template<class T>
+static etcpak_force_inline T SmoothStep( T x )
+{
+    return x*x*(3-2*x);
+}
+
+static etcpak_force_inline uint8_t clampu8( int32_t val )
+{
+    if( ( val & ~0xFF ) == 0 ) return val;
+    return ( ( ~val ) >> 31 ) & 0xFF;
+}
+
+template<class T>
+static etcpak_force_inline T sq( T val )
+{
+    return val * val;
+}
+
+static etcpak_force_inline int mul8bit( int a, int b )
+{
+    int t = a*b + 128;
+    return ( t + ( t >> 8 ) ) >> 8;
+}
+
+#endif