Warn users when their SVGMobject's match the background

[jsonvillanueva]

Enhancement proposal

The recent enhancements to the SVG engine in #915 has allowed colors to be taken from SVG files (whereas before the behavior initialized with a WHITE color). Unfortunately this now leads to confusion when SVG files have a black (#000000) stroke/fill and no longer appear to render... they do, they're just hidden.

Here's an example of a user running into this issue. I've also succumbed to this despite having fully reviewed the SVG engine and was able to provide a solution (knowing that coloring works), so this would mainly serve as a helpful reminder to the end-user to check their SVG for colors matching the Scene's background color. Hopefully this problem won't be as prevalent in manim help channels as a result.

Additional comments

This could be further extended to deal with the more general VMobject; although, VMobject's don't default to BLACK (most SVG editors do default to BLACK). I believe this check should be done only once on initialization of the general Mobject. Doing it for all frames during animations would hinder performance in some way or another -- but might be a good warning for the future with better GPU support.

[markromanmiller]

This is trickier than I thought. The background color is part of Camera, but the item color is the property of the Mobject.