gui_toolkit 0.4.1

Added

• Layers (group opacity): draw.push_layer(dc, bounds, alpha) / draw.pop_layer draw into an offscreen render
  target composited once with alpha, so overlapping primitives of the group do not show through each other (CSS
  opacity). Transforms, rounded / rotated clips, images, text and nested layers work inside. Falls back to
  push_alpha when the layer would be empty or too large. draw.layer_depth.
• Renderer support: render.Layer, Batch.target, frame_submit(..., layers), a pool of fp16 render targets
  (render.acquire_target, release_targets, targets_bytes), render.FLAG_LAYER (texel-exact composite).
• Offscreen renderer: render.init_offscreen(r, size, warp) (no window; WARP software rasterizer available) and
  render.read_pixels. render.destroy releases every GPU object.
• tests/layers: pixel tests (group opacity vs. per-primitive alpha, opaque layer = no layer, nested layers with a
  rotated rounded clip, target pooling, fallback).
• examples/transforms: the orbiting planet fades as one group (push_layer).

Changed

• The shader constant buffer is 32 bytes (adds target_origin). init is split into device creation and pipeline
  setup (shared with init_offscreen); frame_end and resize do nothing for an offscreen renderer.