Add examples/gbm-egl-benchmark.c, a headless benchmark that compares
the display-path cost of uploading Cairo-rendered content to a GL
texture via two methods:
Image surface: glTexSubImage2D (CPU → GPU memcpy)
GBM surface: EGL DMA-BUF import (zero-copy)
The benchmark renders an animated Cairo scene at the requested
resolution, then measures only the upload/import step per frame.
It runs entirely on a DRM render node with no window system required.
Typical results on AMD Radeon RX 580:
1080p (7.9 MB/frame): ~24x faster with GBM (1.42 ms → 0.06 ms)
4K (31.6 MB/frame): ~80x faster with GBM (5.18 ms → 0.07 ms)
Built automatically when gbm, egl, and glesv2 are all available.
