Native bindings to the NV_read_buffer extension.

Unextended OpenGL ES 2.0 only supports using ReadPixels to read from the default color buffer of the currently-bound framebuffer. However, it is useful for debugging to be able to read from non-default color buffers. Particularly, when the NV_draw_buffers extension is supported, each framebuffer may contain multiple color buffers. This extension provides a mechanism to select which color buffer to read from.

The NV_read_buffer extension adds the command ReadBufferNV, which is used to select which color buffer of the currently-bound framebuffer to use as the source for subsequent calls to ReadPixels, CopyTexImage2D, and CopyTexSubImage2D. If the system-provided framebuffer is bound, then ReadBufferNV accepts value BACK. If a user-created FBO is bound, then ReadBufferNV accepts COLOR_ATTACHMENT0. Additionally, if the NV_draw_buffers extension is supported, ReadBufferNV accepts COLOR_ATTACHMENTn_NV (n is 0 to 15).