gui_toolkit 0.7.0

Added

• Retained drawing: draw.Picture, picture_begin / picture_end (record what is drawn between them instead of
  drawing it), replay(dc, pic, m) (Replay_Result: Drawn, Culled, Stale), picture_destroy,
  picture_release, picture_bounds, picture_stale, picture_size, recording. Transforms pushed while
  recording are slots (current_slot, push_slot, picture_set_slot, picture_slot): their matrices change
  without recording again (one GPU range rewritten). Clips pushed while recording are mapped at replay (an exact
  scissor under a uniform scale + translation, as push_clip). A replay draws under the current transform, alpha
  stack and clips, in the order of the frame (immediate drawing and replays mix). Pictures that would show text at
  another raster size, or whose glyphs were evicted from the atlas, report Stale.
• Persistent GPU heaps for pictures (render.Heap_Kind, heap_alloc, heap_free, heap_write, heap_bytes,
  heaps_reset, Slot_Data, Replay, Batch.replay, Frame_Data.replays): first-fit ranges, growth by
  GPU copy. draw.set_retained_budget / retained_bytes: pictures not drawn in the current frame are evicted
  (least recently drawn first) past the budget and uploaded again from their recording when drawn.
• Device loss: render.recover recreates the device, the swap chain or offscreen target, the pipeline and every
  texture under the same handle (contents cleared: device_generation changes); frame_begin and
  draw.begin_frame recover on their own (Present failures and GetDeviceRemovedReason are watched);
  device_lost, simulate_device_loss (tests). The glyph atlas is reset, the heaps emptied (heap_generation).
• Gradients evaluated per pixel: draw.gradient(dc, rect, Gradient, radii, tint); Gradient_Kind (linear,
  radial, conic), repeating, stop positions, transition hints, Gradient_Space (sRGB, linear, Oklab, Oklch, Lab,
  Lch, HSL, HWB), Hue_Method, tiles (size, repeat per axis, spacing: CSS background-size / repeat / space),
  dithering. render.KIND_GRADIENT, gradient tables (Frame_Data.grads, GRAD_*).
• Box shadows: draw.box_shadow (outer, optionally cut out of a rounded rect) and draw.inset_shadow: the
  exact gaussian blur of a rounded rect with per-corner radii (erf along x, the corner arcs integrated over their
  angle), render.KIND_SHADOW, SHADOW_INSET, SHADOW_HOLE.
• Rect_Inst.aux, Clip_Data.xform, CLIP_SCISSOR, Batch.shape.
• Tests: tests/retained (replay identical to direct drawing, slots, z order, partial recording, culling, budget,
  device loss, in both blend spaces), tests/gradients, tests/shadows (against a numeric convolution).
• examples/transforms --retained (with --stress=N).

Changed

• Clip_Data is 96 bytes; render.init and init_offscreen are split (create_swapchain,
  create_offscreen_target) so that recover can rebuild them.
• push_effects / push_layer while recording fall back to the alpha (a picture holds no offscreen layer);
  backdrop_capture returns nothing.