Defer making skshader until we know quality

[mikerreed]

SkImageShader in skia need to know their sampling-options when they are created, and not defer to the (deprecated) filter-quality enum in the SkPaint.

Change the shader object to allow image-shader to defer creating its skia shader until the quality is known.

A deeper change would be to expose sampling options to the Dart API, and have the client provide them directly. This would simplify the current plumbing, remove the unnecessary filter-quality entirely, and give clients more options:

• control over mipmap options
• control over bicubic resampling coefficients (e.g. Mitchell -vs- CatmullRom -vs- custom)

[flutter-dashboard]

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[yjbanov]

@flar I thought you might be the right person to review this, but feel free to unassign if you are not available.