FLT_EPSILON was too small to use when checking if the edge collapse result is an error.
add invert_m3_m3_ex(), invert_m3_ex() functiosn which take an epsilon to check the determinant, saves calculating it twice per edge collapse.
- update face normals when triangulating.
- avoid divide by zero when interpolating customdata on a zero length edge.
- replace zero float comparisons with fabsf() < FLT_EPSILON to avoid numeric error.
also renamed BLI_heap_empty() --> BLI_heap_is_empty() so its obviously readonly function.
