Extracts a texture handle for a specific layer.
No replacement
In the future Cogl may support purely GLSL based layers; for those layers this function which will likely return
COGL_INVALID_HANDLE if you try to get the texture handle from them. Considering this scenario, you should call
get_type first in order check it is of type
TEXTURE before calling this function.
| this |
A |
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a VertexBufferIndices for the texture inside the layer |