Fix (gd): preserve alpha in TrimModifier reference color

[cheack]

Bug

Image::trim() on the GD driver silently no-ops when the image has a transparent margin.

Root cause

TrimModifier::trimColor() averages the four corner pixels' RGB but allocates the imagecropauto() reference color with imagecolorallocate(), which always forces full opacity. When the real corner pixels are transparent, the reference color's alpha never matches them, so imagecropauto() finds nothing within tolerance and crops nothing.

Fix

• Average and preserve alpha across the four corners.
• Allocate the reference color with imagecolorallocatealpha() instead of imagecolorallocate().
• Set imagealphablending(false) / imagesavealpha(true) before sampling, so imagecolorat() returns raw unblended pixel values matching what gets allocated.