[trunk] SVG images are embedded without transparency

Trunk rev. 5283.

In function SvgImage::draw() (file libmscore/image.cpp, line 231), the cache buffer is filled with white background (white is the default parameter for the fill() function):

buffer.fill();

I think this is not entirely correct as SVG images (as well as other types of images) can have a transparent background; by filling the background with a solid colour, any image become non-transparent.

Rather, I think the buffer should be filled with the 'transparent' pseudo-colour:

buffer.fill(Qt::transparent);

A patch is attached.