SPV: Correct selection of storage-image capabilities. Fixes #986.

Code was reflecting an old historical use of sampled as a SPIR-V
2-valued operand, instead of its current 3 values.
diff --git a/Test/baseResults/spv.image.frag.out b/Test/baseResults/spv.image.frag.out
index c21f9bf..d645f2a 100644
--- a/Test/baseResults/spv.image.frag.out
+++ b/Test/baseResults/spv.image.frag.out
@@ -4,10 +4,11 @@
 // Id's are bound by 376
 
                               Capability Shader
-                              Capability SampledRect
-                              Capability Sampled1D
-                              Capability SampledCubeArray
-                              Capability SampledBuffer
+                              Capability StorageImageMultisample
+                              Capability ImageCubeArray
+                              Capability ImageRect
+                              Capability Image1D
+                              Capability ImageBuffer
                               Capability ImageMSArray
                               Capability StorageImageExtendedFormats
                               Capability ImageQuery