feat: add Background colour overload

[guitarrapc]

// white background, black qr
canvas.Render(qr, info.Width, info.Height);

// white background, black qr
canvas.Render(qr, info.Width, info.Height, SKColors.Transparent, SKColors.Black, SKColors.White);

// black background, white qr
canvas.Render(qr, info.Width, info.Height, SKColors.Transparent, SKColors.White, SKColors.Black);

[codecov]

Codecov Report

Merging #46 (01759bd) into main (58cbec7) will increase coverage by 1.66%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #46      +/-   ##
==========================================
+ Coverage   67.62%   69.29%   +1.66%     
==========================================
  Files           7        7              
  Lines        1390     1420      +30     
  Branches      179      180       +1     
==========================================
+ Hits          940      984      +44     
+ Misses        424      410      -14     
  Partials       26       26              

Flag	Coverage Δ
unittests	69.29% <100.00%> (+1.66%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/SkiaSharp.QrCode/QRCodeRenderer.cs	100.00% <100.00%> (ø)
src/SkiaSharp.QrCode/QrCodeExtensions.cs	100.00% <100.00%> (+36.84%)	⬆️

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