Hillshading: make intensity behave like a Z factor (like QGIS)

ℹ️ Funded by an industrial customer.

In QGIS, when enabling hillshading, the Z factor controls the "intensity" of the shading, by artificially exaggerating how the elevation is intepreted, in turn exaggerating the slopes.

This is more correct than our current implementation of hillshading intensity, which acts as a multiplier for the final shading value, and produces very different results beyond 1.