Improve ImGui fractional scaling support

[jaredh]

Summary of the PR

Looking at improving support for Wayland fractional scaling in BlackLabelHQ/RecompOne (issue[1]) has led me to a couple fixes here. Looks like a lot of this is improving in 3.0 with windowing API improvements, but not sure when to expect that to hit GA.

These fixes are reproducible in the ImGui sample app, and seem to stem from deviation from the original C++ port[2][3][4]. The tl;dr is integer division was being performed, truncating fractional values, and never updating the viewport with the framebuffer size. This was resulting in the UI rendering seemingly correct, but mouse input not lining up. Note the cursor position and the selected Configuration row in this screenshot 👇

I've also exposed GetWindowContentScale, which GLFW added at the same time as the monitor scale API; this is required downstream so we can access the fractional scale value from Wayland/mutter.

A platform guard has been added to Silk.NET.Input so I can build this on Linux without the iOS target.

Related issues

[1] BlackLabelHQ/SymphonyRecomp#46
[2] https://github.com/ocornut/imgui/blob/v1.82/backends/imgui_impl_glfw.cpp#L360-L362
[3] https://github.com/ocornut/imgui/blob/v1.82/backends/imgui_impl_opengl3.cpp#L272-L274
[4] https://github.com/ocornut/imgui/blob/v1.82/backends/imgui_impl_opengl3.cpp#L449-L450

Further Comments

I've fixed and tested this against the OpenGL implementation only. I'm not familiar with the status of Vulkan or if it's required there.

[jaredh]

@dotnet-policy-service agree

[jaredh]

When going fullscreen Resize is fired and the updated window size is correctly reflected, but when returning to windowed mode on Wayland, GLFX does not end up invoking the resize callback so we never end up receiving the updated window dimensions.