SL-12865, add bias to alpha mask values to avoid 8-bit acne

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

@@ -230,7 +230,10 @@ void main()
     vec4 diffuse_linear = vec4(srgb_to_linear(diffuse_srgb.rgb), diffuse_srgb.a);
 
 #if (DIFFUSE_ALPHA_MODE == DIFFUSE_ALPHA_MODE_MASK)
-    if (diffuse_linear.a < minimum_alpha)
+
+    // Comparing floats cast from 8-bit values, produces acne right at the 8-bit transition points
+    float bias = 0.001953125; // 1/512, or half an 8-bit quantization
+    if (diffuse_linear.a < minimum_alpha-bias)
     {
         discard;
     }