Restore legacy OpenGL 2.1 support.

glsl/combined.vert

@@ -3,6 +3,21 @@
 uniform vec3 Scroll;
 uniform vec3 r1, r2;
 
+#if __VERSION__ == 120
+attribute vec4 aVertexPosition;
+attribute vec4 aVertexTexCoord;
+attribute vec2 aVertexTexMetadata;
+
+varying vec4 vTexCoord;
+varying vec2 vTexMetadata;
+varying vec4 vChannelMask;
+varying vec4 vDepthMask;
+varying vec2 vTexSampler;
+
+varying vec4 vColorFraction;
+varying vec4 vRGBAFraction;
+varying vec4 vPalettedFraction;
+#else
 in vec4 aVertexPosition;
 in vec4 aVertexTexCoord;
 in vec2 aVertexTexMetadata;
@@ -16,6 +31,7 @@ out vec2 vTexSampler;
 out vec4 vColorFraction;
 out vec4 vRGBAFraction;
 out vec4 vPalettedFraction;
+#endif
 
 vec4 UnpackChannelAttributes(float x)
 {