column: Fix height for width request

Use the width allocated to the child instead of the width allocated to
the column to compute the height requested by the column.

This avoids children whose height depends on their allocated width to be
cropped at the bottom of the column because it doesn't request enough
height.
1 job for column-height-for-width in 1 minute and 55 seconds (queued for 32 seconds)
Status Job ID Name Coverage
  Build
passed #280
librem5
build-debian

00:01:55