Decouple contrast boost from fake gamma.

Replace FakeGamma with FontWeightFlags. Largely mechanical, except for
the changes in ignorePreBlend. With this change, text GMs generally look
the same (or at least more similar) in 8888/srgb/gpu/gpusrgb configs.

No public API changes.
TBR=reed@google.com

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1866293003

Review URL: https://codereview.chromium.org/1866293003
diff --git a/src/ports/SkFontHost_mac.cpp b/src/ports/SkFontHost_mac.cpp
index 53d5610..29b52ca 100644
--- a/src/ports/SkFontHost_mac.cpp
+++ b/src/ports/SkFontHost_mac.cpp
@@ -2030,6 +2030,7 @@
     // All other masks can use regular gamma.
     if (SkMask::kA8_Format == rec->fMaskFormat && SkPaint::kNo_Hinting == hinting) {
 #ifndef SK_GAMMA_APPLY_TO_A8
+        // SRGBTODO: Is this correct? Do we want contrast boost?
         rec->ignorePreBlend();
 #endif
     } else {