op/clip: compute bounds during Path build

The current renderer transforms and processes paths before sending them
to the GPU. It can compute bounds during processing.

The new renderer passes paths verbatim to the GPU, but needs the bounds
for constructing clip bounds.

This change computes the bounds during construction, so it is available
at use. As a bonus for storing the bounds with the path, path caches
(such as for storing text fragments) automatically reuse the bounds
calculations as well.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

gpu/gpu.go

@@ -760,8 +760,7 @@ func (r *renderer) packStencils(pops *[]*pathOp) {
 	*pops = ops
 }
 
-// intersects intersects clip and b where b is offset by off.
-// ceilRect returns a bounding image.Rectangle for a f32.Rectangle.
+// boundRectF returns a bounding image.Rectangle for a f32.Rectangle.
 func boundRectF(r f32.Rectangle) image.Rectangle {
 	return image.Rectangle{
 		Min: image.Point{