Improve SPIR-V handling of RTFlip when running the fuzzer.

If the passed-in shader references RTFlip (i.e., sk_FragCoord is used),
the settings must contain RTFlip layout info; otherwise, an error
occurs. Originally, the fuzzer detected this as a problem because the
error was being delivered via SK_ABORT, but it's failing more cleanly
now that Ethan's new error handling code is in place (causing the fuzzer
to report that the bug was "fixed"). With this CL, the oss-fuzz shader
will actually compile successfully in SPIR-V instead of leading to an
error.

Change-Id: I3268e84bd8e01c95a25ed0845a37324e98033c4b
Bug: oss-fuzz:35916
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/439779
Auto-Submit: John Stiles <johnstiles@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
diff --git a/fuzz/oss_fuzz/FuzzSKSL2SPIRV.cpp b/fuzz/oss_fuzz/FuzzSKSL2SPIRV.cpp
index d5a661d..ba85777 100644
--- a/fuzz/oss_fuzz/FuzzSKSL2SPIRV.cpp
+++ b/fuzz/oss_fuzz/FuzzSKSL2SPIRV.cpp
@@ -15,6 +15,14 @@
     SkSL::Compiler compiler(caps.get());
     SkSL::String output;
     SkSL::Program::Settings settings;
+
+    // This tells the compiler where the rt-flip uniform will live should it be required. For
+    // fuzzing purposes we don't care where that is, but the compiler will report an error if we
+    // leave them at their default invalid values, or if the offset overlaps another uniform.
+    settings.fRTFlipOffset  = 16384;
+    settings.fRTFlipSet     = 0;
+    settings.fRTFlipBinding = 0;
+
     std::unique_ptr<SkSL::Program> program = compiler.convertProgram(
                                                     SkSL::ProgramKind::kFragment,
                                                     SkSL::String((const char*) bytes->data(),