drm: finish the logical-geometry comparison, and chain a re-raise

the pipewire-fallback guard now normalizes BOTH sides to logical before
comparing. last round fixed only the single-display case, which left
the same defect on the shape that actually has it: on a multi-monitor
scaled host the advertised geometry carries the PHYSICAL drm mode plus
the compositor scale, while the portal rect is already logical, so a
scaled output disagreed with itself (2880x1800 against 1440x900) and a
per-connector stream that really was that display was rejected,
leaving it advertised offline instead of degrading. the size check
itself stays: on a multi-monitor host it is what tells one connector
apart from the whole-desktop rect. the failure message reports the
logical numbers, the ones actually compared.

also chains the libdrmtap read failure with `from err` so the original
OSError survives (ruff B904).

build.py

@@ -488,7 +488,7 @@ def _assert_so_has_egl(so_path):
         with open(so_path, 'rb') as f:
             blob = f.read()
     except OSError as err:
-        raise Exception(f'cannot read the built libdrmtap at {so_path}: {err}')
+        raise Exception(f'cannot read the built libdrmtap at {so_path}: {err}') from err
     missing = [m for m in (b'libEGL.so.1', b'eglCreateImageKHR') if m not in blob]
     if missing:
         raise Exception(