Fix XY stage movement after homing

Peter Gabriel Pitrone pointed out that after homing the 4D stage,
pressing the "+" button on the x position would reset the y position to
the one *before* homing.

The culprit is that redundant information is held in too many places
(here: the MM core *and* the GenericXYStage wrapper) and such
information is prone to become inconsistent, even for the best
programmers amongst us.

While the workaround introduced by this commit is far from perfect, it
allows Peter to use the software, at least, while HongKee is working on
cleaning up the software design.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

src/main/java/spim/setup/GenericXYStage.java

@@ -37,7 +37,7 @@ public Device manufacture(CMMCore core, String label) {
 
 	public GenericXYStage() {
 		stageX = stageY = null;
-		destX = destY = 0;
+		destX = destY = -1;
 	}
 
 	public class SubStage extends Stage {
@@ -47,20 +47,17 @@ public SubStage(CMMCore core, String label, boolean isX) {
 			super(core, label);
 
 			iAmX = isX;
-
-			if (isX)
-				GenericXYStage.this.destX = getPosition();
-			else
-				GenericXYStage.this.destY = getPosition();
 		}
 
 		@Override
 		public void setPosition(double pos) {
 			try {
 				if (iAmX) {
+					if (destY < 0) destY = core.getYPosition(label);
 					core.setXYPosition(label, pos, GenericXYStage.this.destY);
 					GenericXYStage.this.destX = pos;
 				} else {
+					if (destX < 0) destX = core.getXPosition(label);
 					core.setXYPosition(label, GenericXYStage.this.destX, pos);
 					GenericXYStage.this.destY = pos;
 				}

src/main/java/spim/setup/PicardXYStage.java

@@ -52,6 +52,10 @@ public void setVelocity(double velocity) throws IllegalArgumentException {
 		public void home() {
 			try {
 				core.home(label);
+				if (iAmX)
+					destX = -1;
+				else
+					destY = -1;
 			} catch (Exception e) {
 				ReportingUtils.logError(e, "Could not home X/Y stage.");
 			}