SPV: Add capability ImageGatherExtended if necessary.

SPIRV/SpvBuilder.cpp

@@ -1464,8 +1464,10 @@ Id Builder::createTextureCall(Decoration precision, Id resultType, bool sparse,
     if (parameters.offset) {
         if (isConstant(parameters.offset))
             mask = (ImageOperandsMask)(mask | ImageOperandsConstOffsetMask);
-        else
+        else {
+            addCapability(CapabilityImageGatherExtended);
             mask = (ImageOperandsMask)(mask | ImageOperandsOffsetMask);
+        }
         texArgs[numArgs++] = parameters.offset;
     }
     if (parameters.offsets) {

Test/baseResults/spv.400.frag.out

@@ -11,6 +11,7 @@ Linked fragment stage:
 
                               Capability Shader
                               Capability Float64
+                              Capability ImageGatherExtended
                               Capability ClipDistance
                               Capability SampledRect
                1:             ExtInstImport  "GLSL.std.450"

Test/baseResults/spv.420.geom.out

@@ -11,6 +11,7 @@ Linked geometry stage:
 
                               Capability Geometry
                               Capability GeometryPointSize
+                              Capability ImageGatherExtended
                               Capability GeometryStreams
                               Capability MultiViewport
                1:             ExtInstImport  "GLSL.std.450"