Optimize CPP code generation.

Previously, we were limiting our calls to appendVAList to 512-byte
chunks to avoid a potential buffer overflow. This had two problems:
1 - it did not avoid the buffer overflow :(
    see chromium:1092743
2 - every call to appendVAList is expensive; it incurs a resize of the
    code buffer (alloc new, memcpy, free old)

This CL removes the 512-byte cap as the buffer overflow issue was
resolved at http://review.skia.org/297276

This CL also includes a few more minor improvements.
- `codeAppendf` now uses a raw string so the gencode is easier to read.
- Optimized `SkStringPrintf("%s", foo)` to `SkString(foo)`.
- Optimized `strA = strB` to `strA.swap(strB)` where safe to do so.

Change-Id: Ia0909a68719848dd2ca655066a9bc6929c8fd09f
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/297358
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
diff --git a/src/gpu/effects/generated/GrComposeLerpEffect.cpp b/src/gpu/effects/generated/GrComposeLerpEffect.cpp
index cacabf7..3a4a323 100644
--- a/src/gpu/effects/generated/GrComposeLerpEffect.cpp
+++ b/src/gpu/effects/generated/GrComposeLerpEffect.cpp
@@ -39,11 +39,12 @@
         } else {
             _sample358 = "half4(1)";
         }
-        fragBuilder->codeAppendf("%s = mix(%s ? %s : %s, %s ? %s : %s, half(%s));\n",
-                                 args.fOutputColor, _outer.child1_index >= 0 ? "true" : "false",
-                                 _sample290.c_str(), args.fInputColor,
-                                 _outer.child2_index >= 0 ? "true" : "false", _sample358.c_str(),
-                                 args.fInputColor, args.fUniformHandler->getUniformCStr(weightVar));
+        fragBuilder->codeAppendf(
+                R"SkSL(%s = mix(%s ? %s : %s, %s ? %s : %s, half(%s));
+)SkSL",
+                args.fOutputColor, _outer.child1_index >= 0 ? "true" : "false", _sample290.c_str(),
+                args.fInputColor, _outer.child2_index >= 0 ? "true" : "false", _sample358.c_str(),
+                args.fInputColor, args.fUniformHandler->getUniformCStr(weightVar));
     }
 
 private: