dav1d_fuzzer: use Dav1dSettings.frame_size_limit instead of a custom picture allocator

Limit frame size in pixels to about 16MP, while allowing the fuzzer to test frame widths and heights above 4096.

This will allow crazy frame sizes like 65535x192 and such, which may be useful to test corner cases, but will also abort on huge frames during frame header parsing instead of during picture allocation like before this change.