fix issues with mask and clip-path attributes

1. Mask attribute now work as described in SVG specs: http://www.w3.org/TR/SVG/masking.html#MaskElement

"....the luminance value is calculated by taking the non-premultiplied RGB color values, applying the luminance-to-alpha coefficients (as defined in the ‘feColorMatrix’ filter primitive) to convert the RGB color values to luminance values.

Finally if the graphics object also includes an alpha channel, then the computed luminance value is multiplied by the corresponding alpha value to produce the mask value. "

2. It's now possible to have both clip-path and mask.