Support for background size

[mgierlasinski]

Currently GradientView renders it's IGradientSource by filling all available space. To handle complex css gradients we should handle css property background-size which says render me using specific rectangle, not whole space. As an initial implementation, if GradientSize < available space, it should be tiled. Later, we can consider adding support for GradientPosition and GradientRepeat (analogical to css: background-position and background-repeat).

API changes

GradientView control

public Size GradientSize { get; set; }

GradientView CSS

background: linear-gradient(...); // no changes, maps to GradientSource
background-size: 100px 100px; // new property, maps to GradientSize