Allow user to set tile size

src/com/sheepit/client/hardware/gpu/GPUDevice.java

@@ -59,5 +59,11 @@ public class GPUDevice {
 	public String toString() {
 		return "GPUDevice [model=" + model + ", memory=" + memory + ", cudaName=" + cudaName + "]";
 	}
+
+	public int getRecommandedTileSize() {
+		// GPU
+		// if the vram is lower than 1G reduce the size of tile to avoid black output
+		return (getMemory() > 1073741824L) ? 256 : 128;
+	}
 	
 }