skin_engine: New tag to draw a rectangle (optionally with a gradient)
%dr(x, y, width, height, [colour1[, colour2]]):
x,y - viewport relative pixel coordinates to start the rectangle.
width, height - obvious. can be '-' to fill the viewport
if both colours are left out the viewports foreground colour will be used
if one colour is specified it will fill the rectangle that colour.
if both colours are specified it will gradient fill the rectangle.
Change-Id: Iad451e99ded663bc7c5d182443659db7d909b388