Fix T51434: Module math operation is wrong in GLSL shading

Based on patch from @lazydodo.

Reviewers: sergey

Differential Revision: https://developer.blender.org/D2661

source/blender/gpu/shaders/gpu_shader_material.glsl

@@ -403,7 +403,7 @@ void math_modulo(float val1, float val2, out float outval)
 
 	/* change sign to match C convention, mod in GLSL will take absolute for negative numbers,
 	 * see https://www.opengl.org/sdk/docs/man/html/mod.xhtml */
-	outval = (val1 > 0.0) ? outval : -outval;
+	outval = (val1 > 0.0) ? outval : outval - val2;
 }
 
 void math_abs(float val1, out float outval)