Fix log_clusters calculation in HarfBuzz-NG code path

The old code wasn't good enough to catch all the glyph (de)composition cases,
thus leading to an assertion in QTextLayout's addNextCluster() helper.

The new code catches all the corner cases and introduces somewhat
better performance to the HB-NG shaper backend.

Change-Id: I5b6c673395a4a039dc55b200abbf74b0ba5d0829
Reviewed-by: Ahmed Saidi <justroftest@gmail.com>
Reviewed-by: Lars Knoll <lars.knoll@digia.com>
Minor optimization for QTextEngine::shapeText()

Remember the engine index for each sub-item and avoid moveGlyphData()
where possible (ie. when there are no glyph indexes to care about).
Also don't memmove data we didn't ever initialize.

Change-Id: Ib8e5fd937a10e4e3c8c0e18961a2e2c1a4167924
Reviewed-by: Ahmed Saidi <justroftest@gmail.com>
Reviewed-by: Lars Knoll <lars.knoll@digia.com>