gtk: fix transparent gaps around split dividers

Fractional scaling could leave transparent pixels between panes and their dividers. Align split allocations to physical pixels with a fixed divider thickness, and keep terminal textures and pointer coordinates aligned with them.

Fit the previous terminal frame to the current allocation during resize. Keep the native separator above both panes so the resize cursor remains centered, and preserve CSS transforms without applying them twice.

AI disclosure: GPT-6 Astra did the heavy lifting. Testing, reviewing by me.