Use transparent rather than white background for ImageMagick SVG thumbnails

Use transparent rather than white background for ImageMagick SVG thumbnails

Default value of $wgSVGConverters['ImageMagick'] now uses transparent
background with white fallback color, rather than just white background.
This is done by using #ffffff00 ("transparent white") as background
color (the PNG file format supports saving color for transparent
pixels).

I could find no historical records for why it was done this way;
possibly the intent was to display white rather than black background
in IE 6 (which doesn't support PNG transparency) or in other contexts
where transparent background is impossible.

Bug: T76475
Change-Id: Ief0c7d71c8cc157e256fc84b1b7be3c23cc5fd9b