Prevent SkMeshSpecifications from being destroyed after main().
Prevent SkMeshSpecifications from being destroyed after main().

Previously, we were storing `sk_sp<SkMeshSpecification>` objects in
static variables. This would cause destructors to execute during
atexit time. A mesh specification contains very complex objects,
including two SkSL::Program objects, which can assert if freed
during program shutdown. (They currently don't because of the
SharedCompiler.)

Change-Id: Iaa635e0232c4554602704c61ffa9fa635fdac251
Bug: skia:13666
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/573262
Reviewed-by: Brian Salomon <61fabaaf0876044aeedfd8e351e485c028d69e7b@google.com>
Auto-Submit: John Stiles <f4fcf42d3bb5924557f1eeb3be66747535e585da@google.com>
Commit-Queue: Brian Salomon <61fabaaf0876044aeedfd8e351e485c028d69e7b@google.com>

google/skia,google/skia,google/skia,google/skia,google/skia,google/skia,google/skia,google/skia,google/skia,google/skia