Front-end: Fix issue #147: ensure layout(index=N) has N in [0,1].

Test/330.frag

@@ -148,3 +148,5 @@ void fooKeyMem()
 {
     KeyMem.precise;
 }
+
+layout(location=28, index=2) out vec4 outIndex2; // ERROR index out of range
\ No newline at end of file

Test/baseResults/330.frag.out

@@ -37,7 +37,8 @@ ERROR: 0:140: 'textureQueryLod' : no matching overloaded function found
 ERROR: 0:140: 'assign' :  cannot convert from 'const float' to 'temp 2-component vector of float'
 ERROR: 0:141: 'textureQueryLod' : no matching overloaded function found 
 ERROR: 0:141: 'assign' :  cannot convert from 'const float' to 'temp 2-component vector of float'
-ERROR: 38 compilation errors.  No code generated.
+ERROR: 0:152: 'index' : value must be 0 or 1 
+ERROR: 39 compilation errors.  No code generated.
 
 
 Shader version: 330
@@ -122,6 +123,7 @@ ERROR: node is still EOpNull!
 0:?     'samp2Ds' (uniform sampler2DShadow)
 0:?     'precise' (global int)
 0:?     'KeyMem' (global structure{global int precise})
+0:?     'outIndex2' (layout(location=28 index=0 ) out 4-component vector of float)
 
 
 Linked fragment stage:
@@ -211,4 +213,5 @@ ERROR: node is still EOpNull!
 0:?     'samp2Ds' (uniform sampler2DShadow)
 0:?     'precise' (global int)
 0:?     'KeyMem' (global structure{global int precise})
+0:?     'outIndex2' (layout(location=28 index=0 ) out 4-component vector of float)
 

glslang/MachineIndependent/ParseHelper.cpp

@@ -4225,6 +4225,13 @@ void TParseContext::setLayoutQualifier(const TSourceLoc& loc, TPublicType& publi
             requireProfile(loc, ECompatibilityProfile | ECoreProfile, "index layout qualifier on fragment output");
             const char* exts[2] = { E_GL_ARB_separate_shader_objects, E_GL_ARB_explicit_attrib_location };
             profileRequires(loc, ECompatibilityProfile | ECoreProfile, 330, 2, exts, "index layout qualifier on fragment output");
+
+            // "It is also a compile-time error if a fragment shader sets a layout index to less than 0 or greater than 1."
+            if (value < 0 || value > 1) {
+                value = 0;
+                error(loc, "value must be 0 or 1", "index", "");
+            }
+
             publicType.qualifier.layoutIndex = value;
             return;
         }