Fix widget selection on high-DPI displays; remove 'hack' shader toggle

[dcwhite]

Fixes #2558

Root cause

The widget-selection render pass draws into an offscreen FBO sized in Qt logical (widget) coordinates, and mouse coordinates are logical too — but the pass never set its own viewport. It inherited whatever was current, which is the physical framebuffer viewport (logical × devicePixelRatio) that Qt sets before every paintGL. On high-DPI displays the selection scene therefore rendered at devicePixelRatio scale relative to the FBO, so the pixel read back at the mouse position missed the widget, breaking shift-click picking.

The "Viewer widget selection correction" preference compensated in the selection vertex shader via a uniform literally named hack:

if(hack) gl_Position.xy = ((gl_Position.xy/gl_Position.w) * vec2(0.5) - vec2(0.5)) * gl_Position.w;

That squeezes NDC into the lower-left quadrant — exactly a devicePixelRatio == 2 correction. Hence the platform dependence:

• Retina Macs (dpr = 2): hack needed, but it defaulted to off → broken picking out of the box
• Standard displays (dpr = 1): hack must stay off
• Fractional scaling (125%/150% on Windows, the default behavior under Qt 6): dpr = 1.25/1.5 → neither checkbox state works, which is why the toggle stopped helping some users

(History: the toggle was added in 0374273 "added toggle selection hack button", Oct 2019, alongside debug printouts of mouse position vs. screen size — a workaround for the symptom rather than the cause.)

Fix

Explicitly set the viewport to the selection FBO's size while rendering the selection pass, and restore the previous viewport afterward (SRInterface::select). Clip space then maps onto the FBO 1:1 at any display scale — integer, fractional, or 1 — so the correction has no reason to exist.

With the root cause fixed, this removes:

• the hack uniform and shader branch in SRInterface.cc
• the widgetSelectionCorrection preference (Preferences.h/.cc)
• the settings persistence entry (Settings.cc) — stale keys in existing user settings files are ignored
• the preferences-dialog checkbox and wiring (Preferences.ui, PreferencesWindow.cc/.h)

Testing

• ✅ macOS (Retina, dpr = 2): shift-click widget picking verified working with the preference removed
• ⬜ Windows at 125%/150% display scaling still needs human testing — this was the configuration the old toggle could not fix, so confirmation there would be valuable
• ⬜ Standard dpr = 1 display (should be unaffected: the explicit viewport matches what was previously inherited)

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