session: plafonne les trames pre-rendues par le pilote NVIDIA

Passe __GL_MaxFramesAllowed=1 a cosmic-comp lorsque /proc/driver/nvidia
existe. Sans cela le pilote laisse grossir sa file de trames pre-rendues,
ce qui ajoute de la latence a l'affichage (smithay#1562, cosmic-comp#1179).

La variable n'est ajoutee que sur machine NVIDIA ; ailleurs l'environnement
transmis reste le seul COSMIC_SESSION_SOCK.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Cargo.lock

@@ -1889,14 +1889,6 @@ dependencies = [
 name = "softbuffer"
 version = "0.4.1"
 
-[[patch.unused]]
-name = "cosmic-freedesktop-icons"
-version = "0.4.0"
-
-[[patch.unused]]
-name = "smithay-clipboard"
-version = "0.8.0"
-
 [[patch.unused]]
 name = "dpi"
 version = "0.1.2"
@@ -1944,3 +1936,11 @@ version = "0.31.0-beta.2"
 [[patch.unused]]
 name = "winit-x11"
 version = "0.31.0-beta.2"
+
+[[patch.unused]]
+name = "cosmic-freedesktop-icons"
+version = "0.4.0"
+
+[[patch.unused]]
+name = "smithay-clipboard"
+version = "0.8.0"