Callnative functions take ScriptContext argument

[ghoulslash]

Adds a struct ScriptContext argument to NativeFunc to allow more convenient scripting with callnative. We can make macros similar to battle scripts with .byte fields afterwards and use those in our callnative function

e.g.

.macro gettilebehaviorat x:req, y:req
    callnative NativeFunc_gettilebehaviorat
    .2byte \x
    .2byte \y
 .endm

void NativeFunc_gettilebehaviorat(struct ScriptContext *ctx)
{
    u16 x    = VarGet(ScriptReadHalfword(ctx));
    u16 y    = VarGet(ScriptReadHalfword(ctx));

    gSpecialVar_Result = MapGridGetMetatileBehaviorAt(x + MAP_OFFSET, y + MAP_OFFSET);
}

[mrgriffin]

I love this.

In case anybody is wondering "will this break people's existing callnatives?" the answer is no: callnative is used from asm, so there's no type checking of the functions (i.e. there's no compile error for callnative-ing a function that doesn't have a ctx parameter)... and that's fine because passing an extra argument (for arguments 1–4) doesn't have any effect on the stack, so existing functions can (and will) just ignore the extra argument.