Add SVG support to Terminal widget as a compile-time option.

[ismail-yilmaz]

Given the increasing popularity of inline images support in virtual terminal emulators, and the usage of SVG in general, the SVG rendering support can be added to TerminalCtrl as a compile-time option, since U++ already provides the necessary and easy-to-use SVG rendering functions.

As for the required changes in the wire protocols: This option doesn't necessarily require any modification of supported/existing wire protocols (DECSIXEL/jexer/iterm2), since TerminalCtrl already uses a generic infrastructure to identify and decode images.

[ismail-yilmaz]

A sort of proof of concept implemention, using the existing jexer protocol:

Works just fine with images.

It uses Upp::RenderSVGImage to render the svg into a raster image that can be displayed as usual. (Currently requires only three more lines of code + 2 lines of code change to the renderer.)

Still, a simple yet fine-grained implementation seems to require at least a few options to the wire protocol (such as default bkg color, custom size, etc.).

@klamonte, would you be okay if I attempt to extend the jexer protocol a little bit to include SVG rendering? (a possible TerminalCtrl-variant?)

@ismail-yilmaz I have no objection. Do you think SVG would make the most sense to be like PNG/JPG, where dimensions are picked up from the image data, or more like RGB where height/width are specified exactly?