gltfpack: Fix UV quantization when material uses texture transforms

Because UVs are stored in a quantized form by default, we need to first
apply the dequantization transform and only then apply the original
texture transform. Our logic was the opposite, and only worked if
quantization offset was 0 or if texture transform was offset-only.

Note that this relies on the original transform being TRS; if the
original transform was TSR, then applying a second non-uniform scale
like so: TSR * TS would introduce skew and the resulting matrix will not
be decomposable.

Currently, three.js is using incorrect order (TSR), but Babylon.js and
PlayCanvas correctly implement glTF specification using TRS.

Author: zeux

gltf/write.cpp

@@ -2,6 +2,7 @@
 #include "gltfpack.h"
 
 #include <float.h>
+#include <math.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
@@ -195,8 +196,11 @@ static void writeTextureInfo(std::string& json, const cgltf_data* data, const cg
 
 	if (qt)
 	{
-		transform.offset[0] += qt->offset[0];
-		transform.offset[1] += qt->offset[1];
+		float rcos = cosf(transform.rotation), rsin = sinf(transform.rotation);
+		float ox = transform.scale[0] * qt->offset[0], oy = transform.scale[1] * qt->offset[1];
+
+		transform.offset[0] += rcos * ox + rsin * oy;
+		transform.offset[1] += -rsin * ox + rcos * oy;
 		transform.scale[0] *= qt->scale[0] / float((1 << qt->bits) - 1) * (qt->normalized ? 65535.f : 1.f);
 		transform.scale[1] *= qt->scale[1] / float((1 << qt->bits) - 1) * (qt->normalized ? 65535.f : 1.f);
 		needs_transform = true;