Calculate raster coords identically to other primitives

[ianthomas23]

Fixes issue #1038, supersedes PR #1042.

Calculation of x, y coords for rasters was using a different code path to that used by other geometric primitives, leading to very small differences (~1e-15) that can cause problems when using tf.stack.

This PR removes the utils.compute_coords function that was only used by Canvas.raster and instead uses the same combination of Axis.compute_scale_and_translation and Axis.compute_index as the other primitives, so that now the coordinates are identical.

I have slightly refactored where this code is called in Canvas.raster as the x and y coords are now independent, and replaced use of np.isclose(...).all() with np.allclose(...) which is simpler.

[philippjfr]

Much better, thank you!

[jbednar]

Fabulous! This is what I was looking for all along. :-)

I keep seeing test jobs being cancelled, but I haven't been cancelling them, and am not sure what is doing that.

[maximlt]

Seems like the timeout of 40 minutes is not enough on MacOs. I'll increase it in #1045.