shaderir: Change the param order of fragment shaders

internal/shaderir/glsl.go

@@ -262,10 +262,10 @@ func (p *Program) glslBlock(b *Block, level int, localVarIndex int) []string {
 			case &p.FragmentFunc.Block:
 				nv := len(p.Varyings)
 				switch {
-				case idx < nv:
-					return fmt.Sprintf("V%d", idx)
-				case idx == nv:
+				case idx == 0:
 					return "gl_FragCoord"
+				case idx < nv+1:
+					return fmt.Sprintf("V%d", idx-1)
 				case idx == nv+1:
 					return "gl_FragColor"
 				default:

internal/shaderir/ir_test.go

@@ -711,20 +711,19 @@ varying vec2 V1;`,
 				FragmentFunc: FragmentFunc{
 					Block: block(
 						[]Type{
-							{Main: Vec2},
-							{Main: Vec4},
 							{Main: Float},
+							{Main: Vec2},
 						},
 						assignStmt(
-							localVariableExpr(5),
+							localVariableExpr(3),
 							localVariableExpr(0),
 						),
 						assignStmt(
-							localVariableExpr(3),
+							localVariableExpr(4),
 							localVariableExpr(1),
 						),
 						assignStmt(
-							localVariableExpr(4),
+							localVariableExpr(5),
 							localVariableExpr(2),
 						),
 					),
@@ -747,12 +746,11 @@ varying float V0;
 varying vec2 V1;
 
 void main(void) {
-	vec2 l0 = vec2(0.0);
-	vec4 l1 = vec4(0.0);
-	float l2 = 0.0;
-	l1 = V0;
-	gl_FragColor = V1;
-	l0 = gl_FragCoord;
+	float l0 = 0.0;
+	vec2 l1 = vec2(0.0);
+	gl_FragColor = gl_FragCoord;
+	l0 = V0;
+	l1 = V1;
 }`,
 		},
 	}

internal/shaderir/program.go

@@ -46,7 +46,8 @@ type VertexFunc struct {
 }
 
 // FragmentFunc takes pseudo params, and the number is len(varyings) + 2.
-// If index == len(varyings), the param represents the coordinate of the fragment (gl_FragCoord in GLSL).
+// If index == 0, the param represents the coordinate of the fragment (gl_FragCoord in GLSL).
+// If index == len(varyings), the param represents (index-1)th verying variable.
 // If index == len(varyings)+1, the param is an out-param representing the color of the pixel (gl_FragColor in GLSL).
 type FragmentFunc struct {
 	Block Block

internal/testing/shader.go

@@ -288,8 +288,8 @@ func ShaderProgramImages(imageNum int) shaderir.Program {
 
 	// In the fragment shader, local variables are:
 	//
-	//   0: Varying variables (vec2)
-	//   1: gl_FragCoord
+	//   0: gl_FragCoord
+	//   1: Varying variables (vec2)
 	//   2: gl_FragColor
 	//   3: Actual local variables in the main function
 
@@ -303,7 +303,7 @@ func ShaderProgramImages(imageNum int) shaderir.Program {
 	}
 	texPos := shaderir.Expr{
 		Type:  shaderir.LocalVariable,
-		Index: 0,
+		Index: 1,
 	}
 
 	var stmts []shaderir.Stmt