Add support for PF4 to PFM loader

[Wouterdek]

This PR adds support for the PF4 extension to the PFM format, as described in issue #109.

Two changes were made:

• Modify the parsing of the magic word in the header to detect PF4 (RGBA) images
• Replace getline with a loop that works for both \r and \n.

Reference images from both the PFM website and the PF4 website now work correctly.

[Tom94]

Thanks a bunch and apologies for the delay. :)