Fixed the mitsuba xml transform parser:
- Before the fix, it only handled a single matrix (used by all the scenes (identity)), or only one S, one T and one R matrix, and not in the right order specified in the scene file.
- It now handles correctly transformations as described in the doc (a sequence of commands that accumulate in the right order).
- It also handles lookat transforms (as specified in the doc).
- Also rewrote the normal transformation part at the end (same result, but "more accurate").