gpu, op, internal/ops: add affine transformations

Add support for affine transformations. The key changes are outlined
below.

- Painting/clipping with rectangles is handled by, for complex
  transforms, creating clipping paths representing the transformed
  rectangle and using a larger bounding box. Cover/Blit shaders updated
  correspondingly to correctly map texture cordinates from the new
  bounding boxes.
- Since path splitting must happen on CPU the transforms must happen CPU
  side as well - offsets removed from shaders.
- Complex transforms will lead to different path splitting which means
  that GPU arrays can no longer be cached if the transform has changed.
  Thus the current transform is added as a key to the cache.
- Add a public API to op for setting Affine transformations.

There are a number of optimizations that could be explored further but
which are left out now:
- Caching also of CPU operations (e.g path splitting & transforms) and
  not only caching the GPU arrays.
- Allow for re-use of cached GPU vertices if the transformation change
  is a pure offset / scaling since the splitting is then the same.

Signed-off-by: Viktor <viktor.ogeman@gmail.com>

internal/ops/ops.go

@@ -16,6 +16,13 @@ type Quad struct {
 	From, Ctrl, To f32.Point
 }
 
+func (q Quad) Transform(t f32.Affine2D) Quad {
+	q.From = t.Transform(q.From)
+	q.Ctrl = t.Transform(q.Ctrl)
+	q.To = t.Transform(q.To)
+	return q
+}
+
 func EncodeQuad(d []byte, q Quad) {
 	bo := binary.LittleEndian
 	bo.PutUint32(d[0:], math.Float32bits(q.From.X))