Fix decimal point bug in Context2D color parsing

strtod() uses the locale's decimal point, which for
me is ','. But we want to always use '.' when parsing
color values for Context2D. qstrtod() does that.

This fixes two test failures in qquickcanvasitem.

Change-Id: I2ea58ad328f26903c57c7c80ed95fd24599805f4
Reviewed-by: Charles Yin <charles.yin@nokia.com>