[fuzz] Raise loop termination limit to 257 in SkSL for fuzzer builds

* Commit 3106fc501d set `kLoopTerminationLimit = 256` under `SK_BUILD_FOR_FUZZER`, which caused loops with exactly 256 iterations to be incorrectly rejected.
* Deserializing an `SkMatrixConvolutionImageFilter` with a large kernel (65..256) compiles the `kMatrixConvTexLg` runtime effect, which loops 256 times and triggers an `SK_ABORT`.
* This CL fixes the bug by raising `kLoopTerminationLimit` to 257 under `SK_BUILD_FOR_FUZZER`, allowing 256-iteration loops while continuing to reject 257+.

bug: b/562761166
Change-Id: Iecbefdf5cf6e28f0a0cf27976c656ebdc6319693
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1402496
Commit-Queue: Cristina Huang <huangcristina@google.com>
Reviewed-by: Alexis Cruz-Ayala <alexisdavidc@google.com>