gpu,internal/ops: decode scene commands directly, not through quads

We're about to let clip.Path use more of the compute renderer features
(lines, cubic béziers). This change prepares the gpu package for reading
one of several commands types, not just the quadratic béziers of before.

The old Quad type is still the basis for the stroking algorithms, but
this change moves it into package gpu which is the only user.

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

op/clip/clip.go

@@ -10,7 +10,6 @@ import (
 	"gioui.org/f32"
 	"gioui.org/internal/byteslice"
 	"gioui.org/internal/opconst"
-	"gioui.org/internal/ops"
 	"gioui.org/internal/scene"
 	"gioui.org/op"
 )
@@ -154,7 +153,7 @@ func (p *Path) Quad(ctrl, to f32.Point) {
 // QuadTo records a quadratic Bézier from the pen to end
 // with the control point ctrl, with absolute coordinates.
 func (p *Path) QuadTo(ctrl, to f32.Point) {
-	data := p.ops.Write(ops.QuadSize + 4)
+	data := p.ops.Write(scene.CommandSize + 4)
 	bo := binary.LittleEndian
 	bo.PutUint32(data[0:], uint32(p.contour))
 	encodeCommand(data[4:], scene.Quad(p.pen, ctrl, to, false))
@@ -395,5 +394,5 @@ func (o Outline) Op() Op {
 }
 
 func encodeCommand(out []byte, cmd scene.Command) {
-	copy(out, byteslice.Slice(cmd[:]))
+	copy(out, byteslice.Uint32(cmd[:]))
 }