[fabracht]

Adds BufferType::EncQp { pitch, data } for VAEncQPBufferType, the per-macroblock QP map that an encoder in CQP mode reads (one byte per macroblock for H.264).

The buffer is two-dimensional: vaCreateBuffer takes the row pitch as size and the number of macroblock rows as num_elements, which iHD requires (it rejects a size that is not 64-byte aligned and allocates size * num_elements). Buffer::new currently passes num_elements = 1 for everything but slice parameters, and Context::id/Display::handle are crate-private, so a caller cannot create this buffer itself. The row count is data.len() / pitch; a zero pitch yields zero rows and the driver's error instead of a panic.

Also replaces the is_err()/unwrap_err() pairs in the Drop impls of Buffer and MappedCodedBuffer with if let Err, which clippy flags in this file.

Tested with iHD 26.1.2 on Tiger Lake (H.264 High, VAEntrypointEncSlice, CQP): a 1680x1050 IDR frame with the left half of the map at QP 51 and the right at 26 decodes at 20.2 dB PSNR (left) and 50.6 dB (right) against a plain QP 26 encode, and shrinks from 160 KB to 62 KB.

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