Revert "Reland "Revert "Reland "GPU support for SkMixers""""

This reverts commit 1263889f3a3ea44db959f043254f4ee9e4014ac9.

Bug: skia:
Change-Id: I647cb228e4a8169f12f4afadaf1db06843e4e5fd
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/206391
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
diff --git a/src/sksl/SkSLCompiler.cpp b/src/sksl/SkSLCompiler.cpp
index 4b9709a..9d4238f 100644
--- a/src/sksl/SkSLCompiler.cpp
+++ b/src/sksl/SkSLCompiler.cpp
@@ -61,6 +61,10 @@
 #include "sksl_pipeline.inc"
 ;
 
+static const char* SKSL_MIXER_INCLUDE =
+#include "sksl_mixer.inc"
+;
+
 namespace SkSL {
 
 Compiler::Compiler(Flags flags)
@@ -1268,6 +1272,13 @@
                                          strlen(SKSL_PIPELINE_STAGE_INCLUDE), *fTypes, &elements);
             fIRGenerator->fSymbolTable->markAllFunctionsBuiltin();
             break;
+        case Program::kMixer_Kind:
+            inherited = nullptr;
+            fIRGenerator->start(&settings, nullptr);
+            fIRGenerator->convertProgram(kind, SKSL_MIXER_INCLUDE, strlen(SKSL_MIXER_INCLUDE),
+                                         *fTypes, &elements);
+            fIRGenerator->fSymbolTable->markAllFunctionsBuiltin();
+            break;
     }
     for (auto& element : elements) {
         if (element->fKind == ProgramElement::kEnum_Kind) {