0.6.1 — storage params blocks, colours in the target's encoding

Fixed

• rsplot: every per-draw params block — the curve pipelines (line,
  markers, caps, fill, error bars), the data-area clear, the image tiles, the
  3D scene pipelines and the volume ray-caster — is bound as a read-only
  storage buffer instead of a uniform. On Windows' DX12 backend with the WARP
  software adapter (a GPU-less VM) every curve in a pass read the last
  curve's uniform, so all curves took the last curve's colour; storage reads
  were per-curve correct. The block layouts are unchanged.
• rsplot: curve, per-vertex, dashed-gap and data-background colours are
  uploaded in the render target's encoding: sRGB bytes for a non-sRGB target
  (egui-wgpu's swapchain), linear for an sRGB one. They were always
  linearised, which on the non-sRGB target drew every colour darker than its
  Color32 and than egui's own legend swatches (#4f9bff showed as
  (20, 84, 255)); PNG export was affected the same way.
• rsplot: the 2D image path uploads its colour texels in the render
  target's encoding too: the colormap LUT and direct-RGBA tiles are
  Rgba8Unorm for a non-sRGB target and Rgba8UnormSrgb for an sRGB one,
  and the colormap's nan_color follows the same rule. They were always
  sRGB-decoded to linear, which on egui-wgpu's non-sRGB swapchain drew every
  colormapped image and RGBA image darker than its colormap (viridis' mid
  teal (33, 145, 140) showed as (4, 71, 66)).
• rsplot: the 3D scene (paint_scene3d, SceneWidget, ScalarFieldView,
  the volume cut plane and iso-surfaces) is rendered into the sRGB variant of
  the surface format and the blit encodes the linear result to sRGB when the
  surface is non-sRGB, so geometry, mesh shading, fog, image layers and the
  background now show at their Color32 values on egui-wgpu's swapchain
  instead of darker. snapshot_scene3d / SceneWidget::snapshot read back
  the sRGB offscreen target, so their PNGs carry sRGB-encoded bytes whatever
  the surface format (they were as dark as the screen).
• rsplot: VolumeRaycaster uploads its voxel colours in the render
  target's encoding — the sRGB bytes as is for egui-wgpu's non-sRGB swapchain
  (unchanged there), linearised for an sRGB target, where the composite used
  to be encoded a second time on store and showed washed out (an opaque
  (255, 128, 0) voxel read back as (255, 188, 0)).