Elliptical Rects

[elcritch]

Extend the core nkRectangle primitive with independent horizontal and vertical radii for each corner. This allows making "elliptical corners" which generalize the circular corners. They can fallback to previous circle forms and the SDF / Shader code is optimized for that case (e.g. rx == ry).

Changes

• Add the common cornerRadiiY field and NfEllipticalCorners flag while preserving existing circular-corner behavior.
• Propagate elliptical corners through Metal, OpenGL/GLES, and Vulkan, including:
  • fills and strokes
  • clipping and rectangle masks
  • drop and inner shadows
  • backdrop blur
• Reuse elliptical rounded rectangles for drawable ellipses, removing the dedicated ellipse shader modes.
• Keep the existing circular shader path whenever every rx == ry.
• Consolidate Metal attribute uploads into reusable aligned upload buffers.
• Add documentation, examples, transfer support, and renderer/backend tests.

Compatibility

Existing nodes remain circular unless NfEllipticalCorners is set. In elliptical mode, corners supplies the X radii and cornerRadiiY supplies the Y radii:

Fig(
  kind: nkRectangle,
  flags: {NfEllipticalCorners},
  corners: [80'u16, 40'u16, 24'u16, 60'u16],
  cornerRadiiY: [28'u16, 64'u16, 12'u16, 36'u16],
)

Setting every X radius to half the rectangle width and every Y radius to half its height produces a filled ellipse.

Fig is now exactly 256 bytes. Circular backend calls remain source-compatible through automatic promotion to equal X/Y axes.

Validation

• nim r tests/tfignodes_helpers.nim
• nim r tests/trenderfragments.nim
• nim r tests/ttransform.nim
• nim c examples/siwin_cell_grid.nim
• siwin_cell_grid measured approximately 119.9 FPS using Metal on an Apple M3 Pro.