render/gles2: provide public API to access GL texture
Prior to this commit, compositors needed to render the texture to an intermediate off-screen buffer using wlr_renderer APIs if they wanted to use a custom rendering path (e.g. render to a 3D scene).
A new wlr_gles2_texture_get_attribs exposes the GL texture target and ID so that compositors can render wlr_textures with their own shaders. An example of a compositor doing so is available at 1.