Tightly pack RGB buffers downloaded from GPU.

GStreamer defaults to 4 byte aligning stride for RGB/BGR whereas
stride == width * bpp is expected by networks. GPU can support this
so override the default stride value when allocating the DMA buffer
for download, and propagate the tightly packed stride downstream.

BUG: 142698697
Change-Id: I0327f746e547f5ad1805f28efcbd9ea0c6512d9f