fix: fullscreen surface oversized when --force-scale-factor is set (#329)

[aki1770-del]

Problem

When --fullscreen and --force-scale-factor=N are combined, the Wayland surface is created at native_width × N × native_height × N instead of the native display resolution. On an 800×480 display with --force-scale-factor=1.3, the surface becomes 1040×624, causing UI to extend off-screen (portions rendered outside the display area).

Reported in #329. Confirmed by two maintainers.

Root cause

wl_output_listener.mode stores native display pixels directly in view_properties_.width/height for the fullscreen case. The rest of the codebase treats view_properties_ as logical DIP and multiplies by current_scale_ to get physical dimensions — so CreateRenderSurface ends up at native_px × scale instead of native_px.

force_scale_factor is for adjusting the Flutter engine's device pixel ratio (DPR) so UI elements appear larger. It does not change the physical display resolution. The wl_surface_set_buffer_scale call receives the float value truncated to an integer (e.g. 1.3 → 1), so any surface buffer larger than native resolution is incorrect.

The xdg_toplevel_listener.configure callback already handles this correctly (next_width_dip = width / current_scale_). This fix makes wl_output_listener.mode consistent.

Fix

Two changes, both guarded by force_scale_factor so the auto-detected HiDPI path (wl_output.scale) is unchanged:

1. wl_output_listener.mode: store display dimensions as logical DIP (width / current_scale_) when force_scale_factor is set, consistent with xdg_toplevel_listener.configure.

2. CreateRenderSurface: use display_max_width_/height_ directly when force_scale_factor is set, avoiding floating-point rounding for non-integer scale values like 1.3.

Before / after

On an 800×480 display with --fullscreen --force-scale-factor=1.3:

	Before	After
Surface created at	1040×624	800×480
OnWindowSizeChanged reports	1040×624 px	800×480 px
Flutter DPR	1.3	1.3
Flutter DIP	800×480	615×369
UI element scale	broken (off-screen)	✓ 1.3×

Non-force_scale_factor path: unchanged.

Fixes #329