Stabilize filename hash

[kofa73]

• Cache stability: filename buffers in module->params were not zero-filled before g_strlcpy, so trailing junk bytes unpredictably shifted piece->hash even when the user reselected the same file. Added dt_strlcpy_to_fixed (memset + g_strlcpy). Applied to colorin, colorout, overlay, rasterfile, watermark (filename and font).
• Watermark legacy_params missing migration: v1–v5 -> v6 left scale_img and scale_svg uninitialised (malloc'd params, comment claimed "default values" but none were ever set). Now set to DT_SCALE_IMG_LARGER / DT_SCALE_SVG_WIDTH (current defaults in dt_iop_watermark_params_t).
• Watermark legacy_params suspicious migration: v2 already has sizeto, but v2 -> v6 hard-coded scale_base = DT_SCALE_MAINMENU_IMAGE. v3/v4/v5 assign scale_base from o->sizeto; now v2->v6 does that, too.

[jenshannoschwalm]

In short - YES!

[TurboGit]

Thanks!