Fix GLSL not showing shading properly on the backside of faces. Now it flips

the normal towards the viewer, seems to give consistent results with blender
internal, cycles, normal maps, etc.

Started from patch #32761 by Vitor Balbio, but changed it to do normal flipping
earlier so it solves all cases.

source/blender/gpu/shaders/gpu_shader_material.glsl

@@ -147,7 +147,7 @@ void geom(vec3 co, vec3 nor, mat4 viewinvmat, vec3 attorco, vec2 attuv, vec4 att
 	normal = -normalize(nor);	/* blender render normal is negated */
 	vcol_attribute(attvcol, vcol);
 	vcol_alpha = attvcol.a;
-	frontback = 1.0;
+	frontback = (gl_FrontFacing)? 1.0: 0.0;
 }
 
 void mapping(vec3 vec, mat4 mat, vec3 minvec, vec3 maxvec, float domin, float domax, out vec3 outvec)
@@ -2116,7 +2116,7 @@ void node_add_shader(vec4 shader1, vec4 shader2, out vec4 shader)
 void node_fresnel(float ior, vec3 N, vec3 I, out float result)
 {
 	float eta = max(ior, 0.00001);
-	result = fresnel_dielectric(I, N, eta); //backfacing() ? 1.0/eta: eta);
+	result = fresnel_dielectric(I, N, (gl_FrontFacing)? eta: 1.0/eta);
 }
 
 /* geometry */
@@ -2139,7 +2139,7 @@ void node_geometry(vec3 I, vec3 N, mat4 toworld,
 	true_normal = N;
 	incoming = I;
 	parametric = vec3(0.0);
-	backfacing = 0.0;
+	backfacing = (gl_FrontFacing)? 0.0: 1.0;
 }
 
 void node_tex_coord(vec3 I, vec3 N, mat4 viewinvmat, mat4 obinvmat,