Make it cheaper to use SPDR in DLLs

[nil-ableton]

When using SPDR across multiple DLLs, it is necessary to share the SPDR_Context between these DLLs.

Having to call a dll function to grab the context seems like a too big overhead and is cumbersome to write.

How about having a hidden global context for SPDR that can be set with an explicit "set_context" function?

This way all the macros don't need to have a first argument anymore as well.