The GIF reader plug-in uses a private instance variable 'decodeThisRow' to
determine whether a given row should be output, based on the region of interest
and subsampling factors set via an ImageReadParam. This variable is incorrectly
set to 'true' at the start of decoding, whereas it could be false if the
source region were set such that the first row is excluded.

  The variable should be set initially using the same logic that is used on
subsequent rows.

  This bug was found using the test/javax/imageio/IRPtest, which will also serve
as the regression test for the fix.