Suppress implicit sign conversion warnings in public API

Suppres implicit sign conversion warnings when compiling with the
-wsign-conversion flag.
E.g. “qqml.h:235:5: warning: implicit conversion changes signedness:
'const int' to 'size_t' (aka 'unsigned long') [-Wsign-conversion]”

Change-Id: I7a51c51609c39f2431acb95febed6bb741f3c09c
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>
Fix AnimatedSprite interpolation when looping and/or reversing.

Ensure we interpolate in the correct direction, and that interpolation
occurs between loops.

Task-number: QTBUG-36339
Task-number: QTBUG-36340
Change-Id: Iccb097e6a9af9fa6697e2ba9ee852be9eb516edc
Reviewed-by: Jan Arve Sæther <jan-arve.saether@digia.com>