Fail gracefully when encountering 'in fragmentProcessor' in raster backend

SkSLSlide was triggering this assert - now it prints an error every
frame, but doesn't crash.

Bug skia:9941
Change-Id: I4c02a89c8d824acc71ab595af99e1df2f5fc980d
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/272639
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Tyler Denniston <tdenniston@google.com>
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
Auto-Submit: Brian Osman <brianosman@google.com>
diff --git a/src/sksl/SkSLByteCodeGenerator.cpp b/src/sksl/SkSLByteCodeGenerator.cpp
index 90a9dbb..c9c69fa 100644
--- a/src/sksl/SkSLByteCodeGenerator.cpp
+++ b/src/sksl/SkSLByteCodeGenerator.cpp
@@ -83,11 +83,12 @@
         }
         case Variable::kGlobal_Storage: {
             if (is_in(var)) {
-                // If you trip this assert, it means the program is using raw 'in' variables. You
+                // If you see this error, it means the program is using raw 'in' variables. You
                 // should either specialize the program (Compiler::specialize) to bake in the final
                 // values of the 'in' variables, or not use 'in' variables (maybe you meant to use
                 // 'uniform' instead?).
-                SkASSERT(false);
+                fErrors.error(var.fOffset,
+                              "'in' variable is not specialized or has unsupported type");
                 return ByteCode::Pointer{0};
             }
             bool isUniform = is_uniform(var);