Avoid accessing instance variables in inline functions

`DPSOperators.h` exports various inline functions which
access the `methods` variable of the `NSGraphicsContext`
class.  This will result in accessing instance variables
across module boundaries if these functions are used in
external libraries.

Accessing instance variables across module boundaries is
not supported when building on Windows using the Visual
Studio toolchain.

This patch addes a `methods` function on `NSGraphicsContext`
which returns the variable, and reworks the inline
functions to use the `methods` function instead of directly
accessing the variable.