[WIP] SVGs: Allow for Aspect variations of scale.

[patridge]

(Work in progress efforts toward issue #44. Not ready to merge.)

This will eventually allow for scaling SVGs in a given rendering size using the various Xamarin.Forms.Aspect enum values:

• AspectFit: proportionally scaled and smaller by "letterboxing" (default)
• AspectFill: proportionally scaled and larger by "cropping"
• Fill: just stretched to fit the available space

Definitely still needed:

• Neither SvgImage nor NControl.SvgImageView seem to respect size requests for Aspect.Fill (e.g., always the full width in the sample page, even when WidthRequest smaller)
• SvgImage doesn't seem to handle rendering properly for Aspect.AspectFill (just flat out wrong in the sample, but maybe it's failing to recenter the rendered result on the final size)
• SvgImage doesn't seem to handle rendering properly for Aspect.AspectFit when size is determined by layout (e.g., omitting WidthRequest)
• ScaleThatFills extension method: failing tests: scaling down doesn't seem to work properly yet.

[patridge]

FWIW, here are some screenshots from the current sample page for reference. (I'll

SvgImageView

• Aspect.AspectFill: pass (appears to work fine)

• Aspect.AspectFit: pass (appears to work fine)

• Aspect.Fill: fail (doesn't size to given WidthRequest)

SvgImage

• Aspect.AspectFill: fail (just totally not working, it may be scaling correctly but not positioning correctly)

• Aspect.AspectFit: pass

• Aspect.Fill: fail (doesn't size to given WidthRequest)