Bump Veldrid.ImGui to v1.69 of ImGui.NET.

Bump version to 4.6.1.

[Demo] Use platform default backend.

[Demo] Use platform default backend.

Update Veldrid.ImGui to 1.67 of dear imgui.

Add missing package descriptions and tags.

Make Veldrid.VirtualReality depend on a stable version of Veldrid.

Bump ImGui version and dependency to 5.66.0.

[ImGui] Lock Veldrid.ImGui's dependencies to 4.5.0.

Change Veldrid.ImGui's version.json public release tag spec.