Add "sample locations" feature to GrProcessor

Adds a "sample locations" feature to GrProcessor. When enabled, this
allows a processor to know inside the shader where all the samples are
located. Also adds various infastructure to query, cache, and identify
multisample data.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1717393002

Review URL: https://codereview.chromium.org/1717393002

1 files changed, 5 insertions, 3 deletions

diff --git a/src/gpu/GrProgramDesc.h b/src/gpu/GrProgramDesc.h
index 63e060ee62..ec6447d622 100644
--- a/src/gpu/GrProgramDesc.h
+++ b/src/gpu/GrProgramDesc.h
@@ -70,9 +70,11 @@ public:
     }
 
     struct KeyHeader {
-        // Set by GrGLShaderBuilder if there are effects that read the fragment position. Otherwise,
-        // 0.
-        uint8_t                     fFragPosKey;
+        // Set to uniquely identify the rt's origin, or 0 if the shader does not require this info.
+        uint8_t                     fSurfaceOriginKey;
+        // Set to uniquely identify the sample pattern, or 0 if the shader doesn't use sample
+        // locations.
+        uint8_t                     fSamplePatternKey;
         // Set to uniquely idenitify any swizzling of the shader's output color(s).
         uint8_t                     fOutputSwizzle;
         uint8_t                     fSnapVerticesToPixelCenters;