The this pointer was always passed to the plugin callback if the calling convention was a thiscall. Even if the plugin author set the this pointer type to ThisPointer_Ignore.
If a function was optimized to only pass one parameter in a register, but still pass other parameters on the stack, save the registers at the correct offset in the buffer.
If the compiler decided to pass an argument in a register on an internal function instead of pushing it on the stack to save time, allow us to specify the register the parameter is going to be in.
DHookAddParam received another parameter to set the register.
The custom return value was lost when calling the original function.
Save and restore our own return value, if we're about to call the original function.
