xup/capture: add dma-buf transport for A2 updates

[FlyGoat]

This is the xorgxrdp side of the matching dma-buf transport series for neutrinolabs/xrdp#3774.

The first commit is protocol cleanup:

• replace the remaining hard-coded xup message, order, metadata, and client-data ids in rdpClientCon.c with the shared symbolic definitions

The second commit adds dma-buf transport for A2 capture:

• add per-monitor dma-buf-backed glamor pixmaps on the xorgxrdp side
• enable this path only when the client advertises XUP_CAPTURE_TRANSPORT_DMABUF, the capture mode is CC_SUF_A2 or CC_GFX_A2, glamor is active, and pixmap import/export is supported
• export the per-monitor pixmaps with glamor_shareable_fd_from_pixmap() and keep the associated width, height, stride, fourcc, and size metadata
• refresh those exported surfaces after client-info and monitor-layout changes, and destroy them on disconnect
• in rdpCaptureSufA2() and rdpCaptureGfxA2(), copy the dirty regions into the per-monitor dma-buf pixmaps, set XRDP_ENC_SOURCE_DMABUF, and skip the old shmfd payload
• emit XUP_ORDER_PAINT_RECT_DMABUF and XUP_ORDER_EGFX_DMABUF with the dma-buf fd plus surface metadata instead of using only the *_SHMFD orders
• keep accel assist disabled while dma-buf transport is active, and fall back to the existing shmfd path when dma-buf transport is not available

With the matching xrdp changes, this lets A2 updates be encoded from imported dma-buf surfaces instead of requiring a round-trip through shared system memory.

This is working for me with the matching xrdp series on Intel xe.