make ReadPNG and ReadJPEG take a filename instead of a FILE

-> read is a bit slower (memory allocation and such) than reading directly from disk.

-> we're not yet ready to accept stdin as input (-- -) because we still need to guess
the file type with GetImageType(). And since we can't rewind on stdin, this will need
a bit more work before being able to read from stdin.

Change-Id: I6491fac4b07d28d1854518325ead88669656ddbf
5 files changed