In a multi GPU scenario, it may be desirable to use different GPUs
for decode and display responsibilities. For example, if a secondary
GPU has better video decoding capabilities.
In such a scenario, we need to initialise a separate context for each
GPU, and use the display context in hwdec_cuda, while passing the
decode context to avcodec.
Once that's done, the actually hand-off between the two GPUs is
transparent to us (It happens during the cuMemcpy2D operation which
copies the decoded frame from a cuda buffer to the OpenGL texture).
In the end, the bulk of the work is around introducing a new
configuration option to specify the decode device.
Philip Langdale
7424651b96