Fix initial rendering and mouse scaling

[jserv]

This commit adds first-run damage marking to ensure screen content is rendered on startup instead of remaining blank until user interaction. For mouse coordinate scaling, this scales EV_ABS coordinates from the device range (0-32767) to actual screen resolution for correct cursor positioning in touchscreen environments.

Close #62

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Summary by cubic

Fix blank initial screen by marking full-screen damage on first run, and scale EV_ABS touch/mouse coordinates to the display resolution for accurate cursor placement.

[cubic-dev-ai]

3 issues found across 2 files

Prompt for AI agents (all 3 issues)

Understand the root cause of the following 3 issues and fix them.


<file name="backend/linux_input.c">

<violation number="1" location="backend/linux_input.c:88">
This hard-codes the absolute-axis range to 32767, so devices with different ABS_X maxima end up with wildly mis-scaled coordinates (e.g., 0–4095 devices only move the cursor across ~12% of the screen). Please scale using the device’s actual ABS info instead of a fixed constant.</violation>

<violation number="2" location="backend/linux_input.c:96">
Scaling ABS_Y by a hard-coded 32767 fails on devices whose reported ABS_Y range is smaller or larger, causing the cursor to stick to the top/bottom of the screen. Please derive the scale factor from the device’s actual ABS info.</violation>
</file>

<file name="backend/fbdev.c">

<violation number="1" location="backend/fbdev.c:208">
Using a static first_run flag here makes the initial damage run only once per process; if the fbdev backend is torn down and reinitialized, the new screen never gets its startup damage and renders blank until user input. Please store this flag in the per-context state instead of a function-static boolean.</violation>
</file>

_{React with 👍 or 👎 to teach cubic. Mention @cubic-dev-ai to give feedback, ask questions, or re-run the review.}

[huaxinliao]

Next weekend, I will verify the latest patch on the Raspberry Pi to ensure that its behavior meets expectations.