Add tests for legacy I/O flattening.

reference/shaders/legacy/fragment/io-blocks.legacy.frag

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+#version 100
+precision mediump float;
+precision highp int;
+
+varying vec4 VertexOut_color;
+varying highp vec3 VertexOut_normal;
+
+void main()
+{
+    gl_FragData[0] = VertexOut_color + VertexOut_normal.xyzz;
+}
+

reference/shaders/legacy/vert/io-block.legacy.vert

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+#version 100
+
+attribute vec4 Position;
+varying vec4 VertexOut_color;
+varying vec3 VertexOut_normal;
+
+void main()
+{
+    gl_Position = Position;
+    VertexOut_color = vec4(1.0);
+    VertexOut_normal = vec3(0.5);
+}
+

shaders/legacy/fragment/io-blocks.legacy.frag

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+#version 310 es
+#extension GL_EXT_shader_io_blocks : require
+precision mediump float;
+
+layout(location = 1) in VertexOut
+{
+   vec4 color;
+   highp vec3 normal;
+} vin;
+
+layout(location = 0) out vec4 FragColor;
+
+void main()
+{
+   FragColor = vin.color + vin.normal.xyzz;
+}

shaders/legacy/vert/io-block.legacy.vert

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+#version 310 es
+#extension GL_EXT_shader_io_blocks : require
+
+layout(location = 0) out VertexOut
+{
+   vec4 color;
+   vec3 normal;
+} vout;
+
+layout(location = 0) in vec4 Position;
+
+void main()
+{
+   gl_Position = Position;
+   vout.color = vec4(1.0);
+   vout.normal = vec3(0.5);
+}