In multiple locations in the codebase hash_murmur3_one_32 is used for
generating hash values.
Whenever a sequence of multiple hash-calculation happens within a single
function, the previous hash value is always used as input in the following
hash-calculations. The only exception is get_cache_multipass, where currently the previous
hash value is overridden without utilizing it.
This patch corrects that.
Adds a FramebufferCache singletion that operates the same way as UniformSetCache.
Allows creating framebuffers on the fly (and keep them cached if re-requested) such as:
```C++
RID fb = FramebufferCache::get_singleton()->get_cache(texture1,texture2);
```
