hdf5_ep

[svn-r26646] Bug fix: HDFFV-8957 h52gif crashes when run against one of our own examples.

Description: h52gif crashed when it was asked to convert a 24bitimage.
Upon viewing the code, it did not prepare to handle images other than 2 dimensions.
It has no concept of multiple planes images.  Further examinations showed past attempts
to fix it ended up removed some abilities (-p or multiple planes, animation, ...) have
been removed but documentation was not updated. Even its online help message still
shows -p is an option.

Solution: added protection code to flag errors if input request is not an
8bit image within size limits.  (I don't have enough knowledge of the GIF
format to fix this tool. All I did was plugging known bug from crashing the
program.)

Tested: h5committest.