Add explicit HiDPI window metrics support

[planetis-m]

This updates the windowing API to expose HiDPI metrics explicitly while keeping the existing logical-coordinate rendering model.

Main changes:

• change ScreenLayout.scaleX/scaleY from int to float32
• add getWindowLayout() so callers can query current logical size and scale
• add WindowMetricsEvent carrying logical width/height plus scaleX/scaleY
• update SDL3 to handle HiDPI internally by rendering in device pixels while keeping logical coordinates above the driver boundary
• update GTK4 to use a HiDPI backing surface while preserving existing logical text/layout behavior
• wire getWindowLayout() through Cocoa, GTK4, SDL2, X11, and WinAPI
• update examples and driver docs to use the new event model

Design

The intent is to keep scaling driver-owned rather than pushing pixel/logical conversions into the upper layer.

That means:

• app code continues to work in logical coordinates
• drivers decide how to map logical units to device pixels
• apps can now react explicitly to runtime window metric changes, including monitor scale changes

Verification

Built successfully with:

• nim c -d:gtk4 examples/todo.nim
• nim c -d:gtk4 examples/hello.nim
• nim c -d:sdl3 examples/hello.nim
• nim c examples/hello.nim

Notes

WindowResizeEvent is still accepted in the examples for compatibility, but WindowMetricsEvent is now the preferred event for window size/scale updates.

[Araq]

Come on, just let WindowResizeEvent do a little more and remove the new WindowMetricsEvent again.