Add a GetNumSamplesFromRegion function which returns the number of samples given a region and step size. Update Terrain Feature Processor to use this function to get the number of samples instead of computing num samples independently.

Signed-off-by: amzn-sj <srikkant@amazon.com>

Code/Framework/AzFramework/AzFramework/Terrain/TerrainDataRequestBus.h

@@ -161,6 +161,10 @@ namespace AzFramework
                 SurfacePointListFillCallback perPositionCallback,
                 Sampler sampleFilter = Sampler::DEFAULT) const = 0;
 
+            //! Returns the number of samples for a given region and step size.
+            virtual AZStd::pair<size_t, size_t> GetNumSamplesFromRegion(const AZ::Aabb& inRegion,
+                const AZ::Vector2& stepSize) const = 0;
+
             //! Given a region(aabb) and a step size, call the provided callback function with surface data corresponding to the
             //! coordinates in the region.
             virtual void ProcessHeightsFromRegion(const AZ::Aabb& inRegion,

Code/Framework/AzFramework/Tests/Mocks/Terrain/MockTerrainDataRequestBus.h

@@ -92,6 +92,8 @@ namespace UnitTest
             ProcessSurfaceWeightsFromListOfVector2, void(const AZStd::span<AZ::Vector2>&, AzFramework::Terrain::SurfacePointListFillCallback, Sampler));
         MOCK_CONST_METHOD3(
             ProcessSurfacePointsFromListOfVector2, void(const AZStd::span<AZ::Vector2>&, AzFramework::Terrain::SurfacePointListFillCallback, Sampler));
+        MOCK_CONST_METHOD2(
+            GetNumSamplesFromRegion, AZStd::pair<size_t, size_t>(const AZ::Aabb&, const AZ::Vector2&));
         MOCK_CONST_METHOD4(
             ProcessHeightsFromRegion, void(const AZ::Aabb&, const AZ::Vector2&, AzFramework::Terrain::SurfacePointRegionFillCallback, Sampler));
         MOCK_CONST_METHOD4(