Add partial blend-mode support to canvas. Fixes #90

[j-h-a]

Support for the following blend-modes has been added to canvas and passed in by the rasterizer.

SKB_BLEND_CLEAR
SKB_BLEND_SRC
SKB_BLEND_DEST
SKB_BLEND_SRC_OVER
SKB_BLEND_DEST_OVER
SKB_BLEND_SRC_IN
SKB_BLEND_DEST_IN
SKB_BLEND_SRC_OUT
SKB_BLEND_DEST_OUT
SKB_BLEND_SOFT_LIGHT

Now 10 of the 27 blend modes are supported.

Example using SRC_IN and SOFT_LIGHT

Before

After

Notes:

• The after image above is still not 100% correct for these flags (from Noto Color Emoji), but I think that is due to a different issue, with the gradients used on those blend-layers.
• Additional optimization is definitely possible.
• Existing Icon behaviour is preserved by passing explicit SRC_OVER blend-mode.

[memononen]

Overall looks good, we should change how the blend functions are created to make life easier for the next one who need to debug the code.

[memononen]

btw, I found the issues for the gradient mismatch.

[memononen]

Thanks for the improvement!