effects/colorpicker: Fix picking colors

Due to the wrong buffer format, the function failed with
GL_INVALID_OPERATION error, which was silently ignored by the effect.
It resulted in the constant phantom color being returned over and over
again.

Co-Authored-By: David Edmundson <kde@davidedmundson.co.uk>
BUG: 454974
FIXED-IN: 5.24.7, 5.25.4, 5.26


(cherry picked from commit df6331b6)