v0.5.0

New sgl-2d features; nothing changes for a game that does not opt in.

• sgl-2d::canvas::Renderer::set_sprite_filter(linear): sprite pages were
  always sampled nearest; linear sampling is now selectable (nearest stays the
  default). Under linear sampling every fragment's sample is clamped half a
  texel inside its sprite's page rect, so atlas neighbours never bleed and
  1:1 sprites and text stay texel-exact. Opt in with
  renderer.set_sprite_filter(true); otherwise no game-code changes are needed.
• sgl-2d::canvas::SpriteInstance::shape, sgl-2d::canvas::Shape,
  sgl-2d::UiFrame::rounded_rect: anti-aliased signed-distance rounded
  rectangles (fill, inner border of a given width and colour, corner radius)
  drawn through the sprite pipeline with the usual z order, clip and rotation,
  one target pixel wide at any UI scale. SpriteInstance::new sets
  shape: None; a SpriteInstance struct literal written without
  ..SpriteInstance::new(..) must add shape: None. Draw one with
  frame.rounded_rect(rect, radius, fill, border_width, border_color).
• sgl-2d::UiFrame::clip(): returns the clip currently applied, so a widget
  that clips its own text can narrow it and restore it instead of replacing an
  enclosing scroll area's clip. No game-code changes are needed.