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

[StageGuard]

Hi, Is any progress on it ?

[igordmn]

Thanks! Sorry for a long review.

It needs small corrections, and it is good to merge.

[MaapuH]

Corrections applied!

[serjsysoev]

AFAIK WebGL textures have no ids, so emscripten provides the mapping for skia. This means that without registering a texture in the emscripten internal map it is not possible to adopt textures created outside skia (e.g. using js interop). Should we include a workaround for this to allow registering external webgl textures?
It probably should be done as a separate PR, just bringing this up as it would be useful for my use case (adopting textures created using js interop).

[igordmn]

This workaround looks like a code that should be called outside of Skia, not in Skia itself. Skia only provides an API which can be used, including requiring an id that can be obtained by the API you provided.