BPFunctions: Clean up math slightly

No need for magic 342 numbers.
BPMemory: Clean up and rename some variables and comments

This is about the register number, not the opcode.
BPFunctions: Remove an unused method
BPFunctions: Remove use of a dumb method

GetPointer serves no purpose.
PixelShaderManager: Fizzle out fog changes when disabled here

This lets us remove a use of GetConfig.
BPFunctions: Remove the rest of GetConfig
BPStructs: Remove calls to SetInterlacedMode when reloading state

SetInterlacedMode is a dummy no-op that does nothing.
BPMemory: Fix "DISPLAYCOPYFILER" typo
BPStructs: Document BPMEM_BP_MASK better
BPStructs: Reindent BPWritten
BPStructs: Flatten out BPWritten
BPStructs: Move LoadBPReg here
BPStructs: Remove another function wrapper
BPStructs: Consistently put the two shared copy args first

And rename them so they make a bit more sense.
Merge pull request #342 from magcius/bp-cleanup

VideoCommon: BP cleanup.