mkstemp patch for the pngalpha device

Russell,

Here's a little patch to use gp_open_scratch_file in pnga_output_page(). On many systems this call 
in more secure than opening a fixed file and also allows concurrent operation of multiple 
instances. This addresses the issue raised in bug 687243.

Ok to commit?

Proposed log message:
Use gp_open_scratch_file() instead of using a fixed filename in the pngalpha device. Part of the 
fix for bug 687243.


 -r