Support for raw opengl textures

[HrX03]

I am currently developing a wayland compositor which needs to send the window opengl textures to flutter. I tried getting the pixels from the texture and then using them for a PixelBufferTexture but there's some middle step that is failing and i think having a direct opengl texture would benefit my project (plus better performance, i don't need to get pixels and then send those, just send the direct texture). As there is already a base FlutterOpenGLTexture i don't think it would be too complicate to introduce and would benefit many people other than me

[HidenoriMatsubayashi]

Thank you for creating the issue. However, it seems duplicated of #157. But, thanks a lot. Please watch #157