Update README feature matrix: viewshed now supports all 4 backends

Author: brendancol

README.md

@@ -225,7 +225,7 @@ In the GIS world, rasters are used for representing continuous phenomena (e.g. e
 | [Hillshade](xrspatial/hillshade.py) | Simulates terrain illumination from a given sun angle and azimuth | ✅️ | ✅️ | ✅️ | ✅️ |
 | [Slope](xrspatial/slope.py) | Computes terrain gradient steepness at each cell in degrees | ✅️  | ✅️  | ✅️ | ✅️  |
 | [Terrain Generation](xrspatial/terrain.py) | Generates synthetic terrain elevation using fractal noise | ✅️ | ✅️ | ✅️ | |
-| [Viewshed](xrspatial/viewshed.py) | Determines visible cells from a given observer point on terrain | ✅️ |  | | |
+| [Viewshed](xrspatial/viewshed.py) | Determines visible cells from a given observer point on terrain | ✅️ | ✅️ | ✅️ | ✅️ |
 | [Min Observable Height](xrspatial/experimental/min_observable_height.py) | Finds the minimum observer height needed to see each cell *(experimental)* | ✅️ | | | |
 | [Perlin Noise](xrspatial/perlin.py) | Generates smooth continuous random noise for procedural textures | ✅️ | ✅️ | ✅️ | |
 | [Bump Mapping](xrspatial/bump.py) | Adds randomized bump features to simulate natural terrain variation | ✅️ | | | |