Fix intermission camera for interpolation

Made P_backupCameraPosition into a method of camera_t.
Interpolate voodoo dolls in Mobj::Think
Bit of streamlining
Reformatting only
Convert slopespan colormap array to a VALLOCATION
Ignored more xcode artifacts.
More C++ cleanup
Global portaltop/bottom are now VALLOCATIONs
Init portalrender.miny to video.height
De-dupe variable names in portal engine

ptop/pbottom are used by static globals in r_things.cpp and I don't like
them getting conflated.
Cleanup and minor refactoring in r_things
Conversion of portal clip arrays to pointers

pwindow_t lists are cleared out when the resolution changes.
Formatting only
Size maskedrange_t clip arrays at video.width size
Raise resolution dimension caps to 32767

MAX_SCREENWIDTH and MAX_SCREENHEIGHT are now set to max window
dimensions supported in Windows and are no longer used to size any
arrays in the renderer.
Using pattern matching against Xcode-motivated ignores.
Add build/ directory to .gitignore
Remove *.xccheckout files

These are files containing VCS metadata and should not be part of the
repository itself.
Add Emacs ignores
Make a new README file, with more relevant information, and AsciiDoc.