Another attempt for T40981, clipping border does not work with GLSL on

ATIs.

This is actually a test to see if this can be enabled on ATI cards.
According to various sources, newer ATI cards supporting GLSL 3.0
support gl_ClippingDistance in shaders, which is the forward compatible
way to do custom clipping.

This fix will bind 6 additional varying variables on ATIs, which may
lead to some shaders not compiling due to limiting out of those
variables, or to performance degradation. Also I do not have an ATI
handy to test.

Having those in mind, this commit may well be reverted later.

Clipping planes are usually 4 (6 is for cube clipping), but making
shaders depend on viewport state is really bad, and would lead to
recompilation, so I took the worst case here to avoid that.
Hopefully driver does some optimization there.

source/blender/gpu/shaders/gpu_shader_vertex.glsl

@@ -2,6 +2,10 @@
 varying vec3 varposition;
 varying vec3 varnormal;
 
+#ifdef CLIP_WORKAROUND
+varying float gl_ClipDistance[6];
+#endif
+
 void main()
 {
 	vec4 co = gl_ModelViewMatrix * gl_Vertex;
@@ -10,9 +14,13 @@ void main()
 	varnormal = normalize(gl_NormalMatrix * gl_Normal);
 	gl_Position = gl_ProjectionMatrix * co;
 
-#ifndef GPU_ATI
+#ifdef CLIP_WORKAROUND
+	int i;
+	for(i = 0; i < 6; i++)
+		gl_ClipDistance[i] = dot(co, gl_ClipPlane[i]);
+#else
 	// Setting gl_ClipVertex is necessary to get glClipPlane working on NVIDIA
 	// graphic cards, while on ATI it can cause a software fallback.
-	gl_ClipVertex = gl_ModelViewMatrix * gl_Vertex; 
+	gl_ClipVertex = co; 
 #endif