(do not) use SK_GAMMA_APPLY_TO_A8 also for Skia text on Windows

This is the Windows variant of the SK_GAMMA_APPLY_TO_A8 disabling,
again I don't quite understand why we need to disable this,
but with this patch Skia text rendering on Windows seems to be
pixel-perfect when compared with the VCL gen backend.

Change-Id: Iafc1e6353430e695dafa67d19eefd968d4f02ae2
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/90696
Tested-by: Jenkins
Reviewed-by: Luboš Luňák <l.lunak@collabora.com>

external/skia/UnpackedTarball_skia.mk

@@ -30,6 +30,7 @@ skia_patches := \
     clang-attributes-warning.patch.1 \
     fontconfig-get-typeface.patch.0 \
     windows-hfont-typeface.patch.0 \
+    windows-text-gamma.patch.0 \
 
 $(eval $(call gb_UnpackedTarball_set_patchlevel,skia,1))
 

external/skia/windows-text-gamma.patch.0

@@ -0,0 +1,28 @@
+--- ./src/ports/SkFontHost_win.cpp.sav	2020-03-18 10:26:52.470184300 +0100
++++ ./src/ports/SkFontHost_win.cpp	2020-03-18 12:08:04.598406700 +0100
+@@ -1215,17 +1215,23 @@
+         // since the caller may require A8 for maskfilters, we can't check for BW
+         // ... until we have the caller tell us that explicitly
+         const SkGdiRGB* src = (const SkGdiRGB*)bits;
++#if defined(SK_GAMMA_APPLY_TO_A8)
+         if (fPreBlend.isApplicable()) {
+             RGBToA8<true>(src, srcRB, glyph, fPreBlend.fG);
+-        } else {
++        } else
++#endif
++        {
+             RGBToA8<false>(src, srcRB, glyph, fPreBlend.fG);
+         }
+     } else {    // LCD16
+         const SkGdiRGB* src = (const SkGdiRGB*)bits;
+         SkASSERT(SkMask::kLCD16_Format == glyph.fMaskFormat);
++#if defined(SK_GAMMA_APPLY_TO_A8)
+         if (fPreBlend.isApplicable()) {
+             RGBToLcd16<true>(src, srcRB, glyph, fPreBlend.fR, fPreBlend.fG, fPreBlend.fB);
+-        } else {
++        } else
++#endif
++        {
+             RGBToLcd16<false>(src, srcRB, glyph, fPreBlend.fR, fPreBlend.fG, fPreBlend.fB);
+         }
+     }