Fix GLFW DPI scale when high-density framebuffers are disabled

[gaborpapp]

This fixes a DPI/content-scale mismatch in the GLFW backend that caused ImGui and app rendering to shift (and sometimes disappear in fullscreen) during resize. Reproducible in the ImGui sample.

WindowImplGlfw::getContentScale() returned the monitor DPI scale even when high-density framebuffers were disabled. Code using toPixels() (including ImGui) then over-scaled coordinates relative to the actual framebuffer size.

Before the current GLFW rewrite, Window::getContentScale() returned 1.0f for GLFW/Linux, which masked the mismatch.

Fix:
Gate GLFW content scale by the app’s high-density setting:

• When isHighDensityDisplayEnabled() is false, return 1.0f
• Otherwise, use glfwGetWindowContentScale() as before

[andrewfb]

This seems right to me - I'd be inclined to merge as-is