Change MDL light direction to world X + Z

to match QS more closely.

Note: QS also quantizes yaw and ignores pitch, while IW doesn't (closer to WinQuake).

Quake/gl_shaders.h

@@ -1127,10 +1127,10 @@ ALIAS_INSTANCE_BUFFER
 "	vec3 lerpedVert = (worldmatrix * vec4(mix(pose1.pos, pose2.pos, inst.Blend), 1.0)).xyz;\n"
 "	gl_Position = ViewProj * vec4(lerpedVert, 1.0);\n"
 "	out_pos = lerpedVert - EyePos;\n"
-"	vec3 shadevector;\n"
-"	shadevector.x = normalize(worldmatrix[0].xyz).x;\n"
-"	shadevector.y = normalize(worldmatrix[1].xyz).x;\n"
-"	shadevector.z = normalize(worldmatrix[2].xyz).x;\n"
+"	// transform world X and Z axes to local space\n"
+"	mat3 orientation = mat3(normalize(worldmatrix[0].xyz), normalize(worldmatrix[1].xyz), normalize(worldmatrix[2].xyz));\n"
+"	orientation = transpose(orientation);\n"
+"	vec3 shadevector = (orientation[0] + orientation[2]) / sqrt(2.0);\n"
 "	float dot1 = r_avertexnormal_dot(pose1.nor, shadevector);\n"
 "	float dot2 = r_avertexnormal_dot(pose2.nor, shadevector);\n"
 "	out_color = clamp(inst.LightColor * vec4(vec3(mix(dot1, dot2, inst.Blend)), 1.0), 0.0, 1.0);\n"