mesa: Add custom get function for SCISSOR_TEST to _mesa_IsEnabledi

Now that the scissor enable state is a bitfield need a custom function
to extract the correct value from gl_context.  Modeled
Scissor.EnableFlags after Color.BlendEnabled.

Signed-off-by: Courtney Goeltzenleuchter <courtney@LunarG.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

src/mesa/main/enable.c

@@ -1136,6 +1136,13 @@ _mesa_IsEnabledi( GLenum cap, GLuint index )
          return GL_FALSE;
       }
       return (ctx->Color.BlendEnabled >> index) & 1;
+   case GL_SCISSOR_TEST:
+      if (index >= ctx->Const.MaxViewports) {
+         _mesa_error(ctx, GL_INVALID_VALUE, "glIsEnabledIndexed(index=%u)",
+                     index);
+         return GL_FALSE;
+      }
+      return (ctx->Scissor.EnableFlags >> index) & 1;
    default:
       _mesa_error(ctx, GL_INVALID_ENUM, "glIsEnabledIndexed(cap=%s)",
                   _mesa_lookup_enum_by_nr(cap));