Pin gsplat==1.5.3

Keep the exact-pin convention (was ==1.4.0) — the Dockerfile extracts
the gsplat version from pyproject.toml with a 'gsplat==' regex to fetch
the matching git tag, which a >= specifier would break.

pyproject.toml

@@ -64,7 +64,7 @@ dependencies = [
     "xatlas",
     "trimesh>=3.20.2",
     "timm==0.6.7",
-    "gsplat>=1.5.3",
+    "gsplat==1.5.3",
     "pytorch-msssim",
     "pathos",
     "packaging",