UPBGE: Don't make mist enable uniform constant.

This uniform is frequently changed between the viewport settings and
in game settings, to avoid recompiling the materials only for it
and has this uniform is a more a flag than a value: this uniform
back to dynamic uniform.

Fix issue: #609.

source/blender/gpu/intern/gpu_material.c

@@ -2068,7 +2068,7 @@ void GPU_shaderesult_set(GPUShadeInput *shi, GPUShadeResult *shr)
 
 	if (!(ma->mode & MA_NOMIST)) {
 		GPU_link(mat, "shade_mist_factor", GPU_builtin(GPU_VIEW_POSITION),
-		         GPU_select_uniform(&GPUWorld.mistenabled, GPU_DYNAMIC_MIST_ENABLE, NULL, ma),
+		         GPU_dynamic_uniform(&GPUWorld.mistenabled, GPU_DYNAMIC_MIST_ENABLE, NULL),
 		         GPU_select_uniform(&GPUWorld.miststart, GPU_DYNAMIC_MIST_START, NULL, ma),
 		         GPU_select_uniform(&GPUWorld.mistdistance, GPU_DYNAMIC_MIST_DISTANCE, NULL, ma),
 		         GPU_select_uniform(&GPUWorld.mistype, GPU_DYNAMIC_MIST_TYPE, NULL, ma),