Use bilinear filtering except when we shouldn't

Use bilinear filtering unless the Drawable is angled by a multiple of 90
degrees and is either a raster image (like a BitmapSkin) or is around
its native resoluion. This makes both bitmaps and vectors look better in
most cases. This is roughly the same logic that Scratch 2.0 used; we may
want to tweak it over time to see if we can make it even better.

src/SVGSkin.js

@@ -81,10 +81,9 @@ class SVGSkin extends Skin {
                 gl.texImage2D(gl.TEXTURE_2D, 0, gl.RGBA, gl.RGBA, gl.UNSIGNED_BYTE, this._svgRenderer.canvas);
                 this._silhouette.update(this._svgRenderer.canvas);
             } else {
+                // TODO: mipmaps?
                 const textureOptions = {
                     auto: true,
-                    mag: gl.NEAREST,
-                    min: gl.NEAREST, /** @todo mipmaps, linear (except pixelate) */
                     wrap: gl.CLAMP_TO_EDGE,
                     src: this._svgRenderer.canvas
                 };