compile error for mode reduction on QuadMeshCurvilinear

#### ALL software version info
(this library, plus any other relevant software, e.g. bokeh, python, notebook, OS, browser, etc should be added within the dropdown below.)

Software Version Info

```plaintext
Datashader 0.18.1
```

#### Description of expected behavior and the observed behavior

The mode reduction works with QuadMeshRectilinear but fails for QuadMeshCurvilinear

#### Complete, minimal, self-contained example code that reproduces the issue

```python
import numpy as np
import xarray as xr
import datashader as dsh
import datashader.transfer_functions as tf

ds = xr.Dataset(
{"foo": (("x", "y"), np.arange(120).reshape(30, 4))},
coords={"x": np.arange(30), "y": np.arange(4)},
).drop_indexes(("x", "y"))

# -----------> commenting out the bottom line fixes it
ds["x"], ds["y"] = xr.broadcast(ds.x, ds.y)

cvs = dsh.Canvas(x_range=(0, 3), y_range=(0, 4))
mesh = cvs.quadmesh(ds, x="x", y="y", agg=dsh.reductions.mode("foo"))
tf.shade(mesh)
```

#### Stack traceback and/or browser JavaScript console output
```
TypingError: Failed in nopython mode pipeline (step: nopython frontend)
Failed in nopython mode pipeline (step: nopython frontend)
Failed in nopython mode pipeline (step: nopython frontend)
Untyped global name '_2': Cannot determine Numba type of

File "", line 2:

During: Pass nopython_type_inference
During: resolving callee type: type(CPUDispatcher())
During: typing of call at (55)

During: resolving callee type: type(CPUDispatcher())
During: typing of call at (118)

During: resolving callee type: type(CPUDispatcher())
During: typing of call at (55)

File "", line 55:

During: Pass nopython_type_inference
During: resolving callee type: type(CPUDispatcher())
During: typing of call at (26)

During: resolving callee type: type(CPUDispatcher())
During: typing of call at (26)

File "", line 26:

During: Pass nopython_type_inference
```

Start by running the minimal Python example from the issue, especially the xr.broadcast call and the QuadMeshCurvilinear mode reduction path. Compare it with the working QuadMeshRectilinear case and the reported Numba traceback; done means the example completes and tf.shade(mesh) works without a compile error.

Written by the indexing model from the issue text.