gpu,op/clip: implement stroked paths

Flat and Square caps are implemented.
Bevel joins are implemented.

Round caps, Round joins and Miter joins are left for another PR.

Signed-off-by: Sebastien Binet <s@sbinet.org>

op/clip/stroke.go

@@ -0,0 +1,23 @@
+// SPDX-License-Identifier: Unlicense OR MIT
+
+package clip
+
+// StrokeStyle describes how a stroked path should be drawn.
+// StrokeStyle zero value draws a Bevel-joined and Flat-capped stroked path.
+type StrokeStyle struct {
+	Cap StrokeCap
+}
+
+// StrokeCap describes the head or tail of a stroked path.
+type StrokeCap uint8
+
+const (
+	// FlatCap caps stroked paths with a flat cap, joining the right-hand
+	// and left-hand sides of a stroked path with a straight line.
+	FlatCap StrokeCap = iota
+
+	// SquareCap caps stroked paths with a square cap, joining the right-hand
+	// and left-hand sides of a stroked path with a half square of length
+	// the stroked path's width.
+	SquareCap
+)