Add "sharpen" option to SkSL, to LOD bias all textures

This adds a fixed bias (-0.5) to the computed LOD of all
mip-mapped texture fetches. (Technically, to all texture
fetches, but that only matters for mip-mapped ones).

Clients can opt-in with a new GrContextOption.

Bug: skia:7541
Bug: chromium:562162
Change-Id: Ie3cd0679c4ab66f62d2dc32e7e68e5c99355115e
Reviewed-on: https://skia-review.googlesource.com/106322
Reviewed-by: Brian Salomon <bsalomon@google.com>
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
diff --git a/src/gpu/GrContext.cpp b/src/gpu/GrContext.cpp
index 8a5417e..09985e6 100644
--- a/src/gpu/GrContext.cpp
+++ b/src/gpu/GrContext.cpp
@@ -255,6 +255,7 @@
                                                         options));
 
     fDisableGpuYUVConversion = options.fDisableGpuYUVConversion;
+    fSharpenMipmappedTextures = options.fSharpenMipmappedTextures;
     fDidTestPMConversions = false;
 
     GrPathRendererChain::Options prcOptions;