Explore ShadedRect/ShadedPoly for color gradients

[deeplook]

After looking at reportlab.graphics.widgets.grids it seems that ReportLab started at some point to provide ShadedRect and ShadedPoly classes to support color gradients. While these do not seem to render in PDF using its higher level abstractions it does certainly help to render them also in bitmaps, say. It would be very useful to evaluate their potential for filling one import missing functionality in svglib. And although the shading appears to be linear only, even that already would be very useful to have. Thoughts?

[deeplook]

@claudep Maybe you feel tempted to have a look?

[claudep]

There are also linearGradient and radialGradient in pdfgen/canvas.py. However we may have to support clipPath first (not sure).