Add texture `isAllocated()` check before drawing surface

See issue #37

diff --git a/src/Surfaces/QuadSurface.cpp b/src/Surfaces/QuadSurface.cpp
index 3a9089b..e8af961 100644
--- a/src/Surfaces/QuadSurface.cpp
+++ b/src/Surfaces/QuadSurface.cpp
@@ -74,6 +74,10 @@ void QuadSurface::draw(){
 		ofLogWarning("QuadSurface") << "Source texture empty. Not drawing.";
 		return;
 	}
+
+	if(!source->getTexture()->isAllocated()){
+		return;
+	}

 	/*if(mesh.haveVertsChanged() || mesh.haveTexCoordsChanged()){
 	    calculate4dTextureCoords();
diff --git a/src/Surfaces/TriangleSurface.cpp b/src/Surfaces/TriangleSurface.cpp
index 27e3951..88170ce 100644
--- a/src/Surfaces/TriangleSurface.cpp
+++ b/src/Surfaces/TriangleSurface.cpp
@@ -47,6 +47,10 @@ void TriangleSurface::draw(){
 		ofLogWarning("TriangleSurface") << "Source texture is empty. Not drawing.";
 		return;
 	}
+
+	if(!source->getTexture()->isAllocated()){
+		return;
+	}

 	source->getTexture()->bind();
 	mesh.draw();

src/Surfaces/QuadSurface.cpp

@@ -74,6 +74,10 @@ void QuadSurface::draw(){
 		ofLogWarning("QuadSurface") << "Source texture empty. Not drawing.";
 		return;
 	}
+	
+	if(!source->getTexture()->isAllocated()){
+		return;
+	}
 
 	/*if(mesh.haveVertsChanged() || mesh.haveTexCoordsChanged()){
 	    calculate4dTextureCoords();

src/Surfaces/TriangleSurface.cpp

@@ -47,6 +47,10 @@ void TriangleSurface::draw(){
 		ofLogWarning("TriangleSurface") << "Source texture is empty. Not drawing.";
 		return;
 	}
+	
+	if(!source->getTexture()->isAllocated()){
+		return;
+	}
 
 	source->getTexture()->bind();
 	mesh.draw();