Fix window resize stretching the canvas instead of resizing the sketch surface (#127)

[arrxy]

Fix: #127

Both bugs stem from the same root confusion: logical vs. physical pixels. @catilac

Summary

Two rendering issues with a shared cause in how window dimensions were
interpreted.

Fix 1 Resize stretched the canvas instead of reflowing it

Root cause: Bevy's WinitPlugin is disabled (we use a GLFW backend), so
nothing emitted WindowResized. Without that event:

• bevy_render's camera_system never ran on resize
• the camera's Projection stayed pinned to the original size
• the render target was simply scaled to fit — the "stretch"
• width/height (SurfaceSize) were left stale

Fix: sync_to_surface now emits WindowResized itself whenever a window's
physical size actually changes.

Fix 2 macOS Retina rendered 400×400 sketches as 800×800

Root cause: the camera's ProcessingProjection (the sketch's coordinate
space) was built from physical dimensions (width × scale_factor = 800 on a
2× display) instead of the logical size (400), doubling everything.

Fix: use logical width/height for the projection and SurfaceSize;
reserve physical dimensions for the GPU texture (Extent3d) and readback buffer
only.