Also moved both the reader and writer class to the same files, named
files.{cpp,h}

Rows in a bitmap are now padded to be a multiple of 4 bytes long

Implemented padding scalines to 4 bytes if they aren't already that

Also improved error handling slightly

This is so I can test PNG writing by reading a BMP and writing it to a
PNG, effectively the reverse of the PNG reading testing

All functions and classes are now behind the cpp namespace `TehImage`

Had to restrict images to 8 bit depth sadly
This could potentially be changed in the future by setting bit depth
with templates and changing the data to be the specified bit depth

Mainly just cleaning up code

Also added the bitmap image subclass
note: this is pretty hacky in how it works