Workbench: Matcaps: Fix matcap under perspective.

This fixes the problem that matcaps have when using a very larg FOV in
perspective view.

This was because it was stupidly using the normal direction which can not
aligned with the view vector under perspective.

So to workaround this problem we don't use the normal as is, but compute
how much it's facing the camera.

This changes how matcaps looks in perspective because they now always use
the full range of the matcap (which is expected).

source/blender/draw/engines/workbench/shaders/workbench_common_lib.glsl

@@ -79,3 +79,16 @@ vec3 view_vector_from_screen_uv(vec2 uv, vec4 viewvecs[3], mat4 proj_mat)
 	             : vec3(0.0, 0.0, 1.0);
 }
 
+vec2 matcap_uv_compute(vec3 I, vec3 N, bool flipped)
+{
+	/* Quick creation of an orthonormal basis */
+	float a = 1.0 / (1.0 + I.z);
+	float b = -I.x * I.y * a;
+	vec3 b1 = vec3(1.0 - I.x * I.x * a, b, -I.x);
+	vec3 b2 = vec3(b, 1.0 - I.y * I.y * a, -I.y);
+	vec2 matcap_uv = vec2(dot(b1, N), dot(b2, N));
+	if (flipped) {
+		matcap_uv.x = -matcap_uv.x;
+	}
+	return matcap_uv * 0.496 + 0.5;
+}

source/blender/draw/engines/workbench/shaders/workbench_deferred_composite_frag.glsl

@@ -67,11 +67,9 @@ void main()
 	vec3 I_vs = view_vector_from_screen_uv(uv_viewport, viewvecs, ProjectionMatrix);
 
 #ifdef STUDIOLIGHT_ORIENTATION_VIEWNORMAL
-	vec2 matcap_uv = normal_viewport.xy / 2.0 + 0.5;
-	if (world_data.matcap_orientation != 0) {
-		matcap_uv.x = 1.0 - matcap_uv.x;
-	}
-	diffuse_color = texture(matcapImage, matcap_uv);
+	bool flipped = world_data.matcap_orientation != 0;
+	vec2 matcap_uv = matcap_uv_compute(I_vs, normal_viewport, flipped);
+	diffuse_color = textureLod(matcapImage, matcap_uv, 0.0);
 #endif
 
 #ifdef V3D_SHADING_SPECULAR_HIGHLIGHT

source/blender/draw/engines/workbench/shaders/workbench_forward_transparent_accum_frag.glsl

@@ -43,7 +43,9 @@ void main()
 	vec3 I_vs = view_vector_from_screen_uv(uv_viewport, viewvecs, ProjectionMatrix);
 
 #ifdef V3D_LIGHTING_MATCAP
-	diffuse_light = texture(matcapImage, normal_viewport.xy / 2.0 + 0.5).rgb;
+	bool flipped = world_data.matcap_orientation != 0;
+	vec2 matcap_uv = matcap_uv_compute(I_vs, normal_viewport, flipped);
+	diffuse_light = texture(matcapImage, matcap_uv).rgb;
 #endif
 
 #ifdef V3D_SHADING_SPECULAR_HIGHLIGHT