API Cleanup: Use PixelSize for device-dependent sizes.

[grokys]

This is something I've been meaning to do for years... Our API was inconsistent about accepting pixel sizes and DPI. For example:

• RenderTargetBitmap accepted a DPI but WriteableBitmap didn't
• IBitmap didn't expose the DPI so once it was put into RenderTargetBitmap it couldn't be read again
• Sometimes DPI was represented by a Vector and sometimes by a pair of doubles
• etc.

This PR introduces a new struct PixelSize which has an int width and height, and also methods for converting to/from a Size with a specified DPI (instead of having to do size = new Size(width / (dpi / 96, height / (dpi / 96)); which isn't so easy to remember). Hopefully this helps make the distinction between device-independent pixels and device pixels clearer.

Wherever a pixel size is passed to a constructor to create a bitmap, a DPI is also passed as a Vector.

Unfortunately Skia doesn't provide an API for getting an image's DPI, so images loaded there are always 96dpi. Not sure what to do about this. For this reason Image still always uses the pixel size as the size to render, rather than the pixel size scaled by DPI.

[jkoritzinsky]

Contributes to #1292.

[ncarrillo]

This looks good to me. Hooray for using types to convey units :).

[jmacato]

This is yet another breaking change that should be added to wiki :)