add BackendTexture and Image.adoptTextureFrom

[MaapuH]

New:

(related #1019)

1. Adds bindings to GrBackendTextures::MakeGL
2. Adds bindings to SkImages::AdoptTextureFrom
3. Adds bindings to GrBackendTextures::GLTextureParametersModdified

Changes in build scripts:

(related #742)

1. Add file paths treating if building on Windows machine
2. Use "emcc.bat" instead of "emcc" if building on Windows machine, as described on Emscripten website

Use-case:

Allows skia to draw images, rendered externally on gpu.
Compose would benefit greatly, allowing to embed VideoPlayer or Browser without any friction or need to use Swing.

[MaapuH]

Besides already implemented above, at the moment I am working on these new bindings:
DirectContext.createBackendTexture() // this is common for all backends
DirectContext.deleteBackendTexture() // this is common for all backends

BackendTexture.backend // allows to check which backend is texture created on
BackendTexture.getGLInfo // allows to get id of the texture if the backend is GL, this we can use for external rendering

We can also getDirect3DInfo for DirectX backend, getMetalInfo for Metal, etc
These I'll be doing when I'm done with GL bindings

I will submit another PR when all the upcoming bindigs are implemented