cut SK_SAMPLES_FOR_X, simplify SK_?32_SHIFT

SK_SAMPLES_FOR_X doesn't do anything other than set the SK_?32_SHIFT
macros, so we can just set them instead, and further we can derive
everything we need from SK_R32_SHIFT.

Change-Id: I766d00c868da04939208353d09201f189a99c0c7
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/210121
Commit-Queue: Hal Canary <halcanary@google.com>
Reviewed-by: Hal Canary <halcanary@google.com>
Auto-Submit: Mike Klein <mtklein@google.com>
diff --git a/BUILD.gn b/BUILD.gn
index 3fe0dc6..92ab115 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -109,7 +109,7 @@
     defines += [ "SKIA_DLL" ]
   }
   if (is_fuchsia || is_linux) {
-    defines += [ "SK_SAMPLES_FOR_X" ]
+    defines += [ "SK_R32_SHIFT=16" ]
   }
   if (skia_enable_flutter_defines) {
     defines += flutter_defines