Make QUrl::isLocalFile fast by storing a flag

The XDG specification for file URIs requires us to use triple slashes in
file:/// for URLs with absolute paths. I don't like special-casing any
particular scheme, but we've done it for file for many years now.

Since we need to test this situation in a couple of places, it's easier
to just cache the result once, in setScheme (both functions).

Change-Id: I078b45b5b6c861f4caee082b4730fd6f67684ae4
Reviewed-by: David Faure <david.faure@kdab.com>
Fix assert when converting RGBx8888 to ARGB32_PM

RGBx8888 to ARGB32_PM is incorrectly using the RGBA8888 to ARGB32_PM
which asserts the input format is RGBA8888. Since the routine also
performs an unnecessy premul, we should be using a the generic rgba2argb
routine.

Change-Id: I7b67328f804f5f2a9664a35c04836679e8c8b8e5
Reviewed-by: Gunnar Sletta <gunnar.sletta@jollamobile.com>