VideoToolbox use different identifiers for the same pixel format
with different color range, like

kCVPixelFormatType_420YpCbCr8BiPlanarVideoRange
kCVPixelFormatType_420YpCbCr8BiPlanarFullRange.

Before the patch, vt_pool_alloc() always use limited range, and it
will fail for pixel format AV_PIX_FMT_BGRA since there is no limited
range kCVPixelFormatType_32BGRA.
Signed-off-by: Zhao Zhili <zhilizhao@tencent.com>