layers: Add usage bit validation for buffer descriptors

Verify that buffer used to update all buffer descriptor types (texel or
general) has the correct usage bits set.
descriptor
diff --git a/layers/descriptor_sets.h b/layers/descriptor_sets.h
index a3cf28d..a211d2f 100644
--- a/layers/descriptor_sets.h
+++ b/layers/descriptor_sets.h
@@ -346,6 +346,7 @@
   private:
     bool VerifyWriteUpdateContents(const VkWriteDescriptorSet *, const uint32_t, std::string *) const;
     bool VerifyCopyUpdateContents(const VkCopyDescriptorSet *, const DescriptorSet *, const uint32_t, std::string *) const;
+    bool ValidateBufferUpdate(VkBuffer, VkDescriptorType, std::string *) const;
     // Private helper to set all bound cmd buffers to INVALID state
     void InvalidateBoundCmdBuffers();
     bool some_update_; // has any part of the set ever been updated?