Fix OpenGL on Android after adding optional depth fog

[dsnopek]

After PR #84792 was merged, I'm getting this error on Android (I'm testing on the Meta Quest 3) with the OpenGL renderer:

02-21 09:51:43.351 24292 24339 E godot   : USER ERROR: SceneShaderGLES3: Fragment shader compilation failed:
02-21 09:51:43.351 24292 24339 E godot   : ERROR: 0:989: '-' :  wrong operand types  no operation '-' exists that takes a left-hand operand of type 'const int' and a right operand of type 'float' (or there is no acceptable conversion)
02-21 09:51:43.351 24292 24339 E godot   : ERROR: 0:989: 'assign' :  cannot convert from 'const int' to 'float'
02-21 09:51:43.351 24292 24339 E godot   : ERROR: 2 compilation errors.  No code generated.

This PR fixes that!

[akien-mga]

Thanks!