indexgenerator: Fix index_count assertion in generateVertexRemapMulti

In generateVertexRemap and generateVertexRemapCustom, we permit the use
on non-indexed data if the number of elements is not divisible by 3.
This does not affect the algorithm; the assertion is there to flag
accidental incorrect uses, and this change aligns the assertion between
all three function variants.

Author: zeux

src/indexgenerator.cpp

@@ -325,7 +325,7 @@ size_t meshopt_generateVertexRemapMulti(unsigned int* destination, const unsigne
 	using namespace meshopt;
 
 	assert(indices || index_count == vertex_count);
-	assert(index_count % 3 == 0);
+	assert(!indices || index_count % 3 == 0);
 	assert(stream_count > 0 && stream_count <= 16);
 
 	for (size_t i = 0; i < stream_count; ++i)