Fix missing basis orthogonalizations in GLSL backend (#1177)

I've encountered shading artifacts in normal mapped MaterialX materials, and upon closer inspection I believe that these are caused by the missing basis orthogonalizations @Tellusim brought to attention in PR #1049.

However, instead of using a second normalize(cross(..)) call, I've applied the Gram-Schmidt algorithm.

libraries/pbrlib/genglsl/lib/mx_environment_fis.glsl

@@ -13,8 +13,8 @@ float mx_latlong_compute_lod(vec3 dir, float pdf, float maxMipLevel, int envSamp
 vec3 mx_environment_radiance(vec3 N, vec3 V, vec3 X, vec2 alpha, int distribution, FresnelData fd)
 {
     // Generate tangent frame.
-    vec3 Y = normalize(cross(N, X));
-    X = cross(Y, N);
+    X = normalize(X - dot(X, N) * N);
+    vec3 Y = cross(N, X);
     mat3 tangentToWorld = mat3(X, Y, N);
 
     // Transform the view vector to tangent space.

libraries/pbrlib/genglsl/mx_conductor_bsdf.glsl

@@ -11,7 +11,8 @@ void mx_conductor_bsdf_reflection(vec3 L, vec3 V, vec3 P, float occlusion, float
 
     N = mx_forward_facing_normal(N, V);
 
-    vec3 Y = normalize(cross(N, X));
+    X = normalize(X - dot(X, N) * N);
+    vec3 Y = cross(N, X);
     vec3 H = normalize(L + V);
 
     float NdotL = clamp(dot(N, L), M_FLOAT_EPS, 1.0);

libraries/pbrlib/genglsl/mx_dielectric_bsdf.glsl

@@ -9,7 +9,8 @@ void mx_dielectric_bsdf_reflection(vec3 L, vec3 V, vec3 P, float occlusion, floa
 
     N = mx_forward_facing_normal(N, V);
 
-    vec3 Y = normalize(cross(N, X));
+    X = normalize(X - dot(X, N) * N);
+    vec3 Y = cross(N, X);
     vec3 H = normalize(L + V);
 
     float NdotL = clamp(dot(N, L), M_FLOAT_EPS, 1.0);

libraries/pbrlib/genglsl/mx_generalized_schlick_bsdf.glsl

@@ -9,7 +9,8 @@ void mx_generalized_schlick_bsdf_reflection(vec3 L, vec3 V, vec3 P, float occlus
 
     N = mx_forward_facing_normal(N, V);
 
-    vec3 Y = normalize(cross(N, X));
+    X = normalize(X - dot(X, N) * N);
+    vec3 Y = cross(N, X);
     vec3 H = normalize(L + V);
 
     float NdotL = clamp(dot(N, L), M_FLOAT_EPS, 1.0);