Don't rely on GLSL 1.40 (GL3.1)

Certain(?) Intel chipsets don't support anything above GL3.0 in a compatibility
profile, but support GL3.3 in core profiles. For now, it's enough to use GLSL
1.30 with GL_ARB_draw_instanced.

Author: kcat

data/shaders/terrain.frag

@@ -1,4 +1,4 @@
-#version 140
+#version 130
 
 uniform vec4 illumination_color;
 

data/shaders/terrain.vert

@@ -1,4 +1,5 @@
-#version 140
+#version 130
+#extension GL_ARB_draw_instanced : enable
 
 uniform mat4 osg_ModelViewProjectionMatrix;
 uniform mat4 osg_ModelViewMatrix;
@@ -17,8 +18,8 @@ flat out uint TexIndex;
 
 void main()
 {
-    int x = gl_InstanceID & 15;
-    int y = gl_InstanceID / 16;
+    int x = gl_InstanceIDARB & 15;
+    int y = gl_InstanceIDARB / 16;
     vec4 pos = osg_Vertex + vec4(x*256, 0, y*-256, 0);
 
     gl_Position = osg_ModelViewProjectionMatrix * pos;