[BUG-232834][BUG-232871] Alpha mask is being applied with emissive mask

This is the most local fix for this issue, addressing the specific unqualified use of HAS_ALPHA_MASK.
If we find other issues with alpha mask being applied incorrectly then, it may be better to fix higher up in llvieweshadermgr.cpp by reverting the changes from SL-17532. 
For now, this way works for this specific bug without non-emissive side-effects.

indra/newview/app_settings/shaders/class1/deferred/materialF.glsl

@@ -227,7 +227,7 @@ void main()
     vec4 diffcol = texture2D(diffuseMap, vary_texcoord0.xy);
 	diffcol.rgb *= vertex_color.rgb;
 
-#ifdef HAS_ALPHA_MASK
+#if HAS_ALPHA_MASK && (DIFFUSE_ALPHA_MODE != DIFFUSE_ALPHA_MODE_EMISSIVE)
 #if DIFFUSE_ALPHA_MODE == DIFFUSE_ALPHA_MODE_BLEND
     if (diffcol.a*vertex_color.a < minimum_alpha)
 #else