Previously, we checked if the bounding box of any child primitives
extended outside the rectangle of the allocated task rect. However,
there is a simpler way to calculate this.

If the allocated size of the render task is >= the unclipped size
of the picture bounding rect, no scissor is needed, since we know
that local clip rects will take care of ensuring nothing is
drawn outside the task boundary.

This is an optimization, but the main benefit is removing one more
piece of code that relies on knowledge of screen / device rects,
which simplifies the ongoing work to be able to rasterize in other
coordinate systems.