[do-not-review] Export tangram as a library

[mathisloge]

This is based on #2274 and #2272

What this PR does is to add export targets to all dependencies and tangram-core. This should enable anyone to consume tangram as a normal lib.

As an example, I've added the resulting debug installation.

The install logic is completly in https://github.com/tangrams/tangram-es/commit/8b0940bbded94bae43b33baf023eaed5591f44b1

install.zip

As a next step, the platforms could be seperated as own targets which can be switched on via the already existing TANGRAM_PLATFORM variable.