OWScatterPlotGraph: Fix compute_sizes/symbols
OWScatterPlotGraph: Fix compute_sizes/symbols

When computing the size/shape data use the 'valid_data' mask to filter
out points with invalid x, y coordinates.

qPCR4vir/orange3,cheral/orange3,qPCR4vir/orange3,marinkaz/orange3,kwikadi/orange3,qPCR4vir/orange3,cheral/orange3,cheral/orange3,kwikadi/orange3,qPCR4vir/orange3,cheral/orange3,qPCR4vir/orange3,cheral/orange3,kwikadi/orange3,marinkaz/orange3,marinkaz/orange3,cheral/orange3,qPCR4vir/orange3,kwikadi/orange3,marinkaz/orange3,kwikadi/orange3,kwikadi/orange3,marinkaz/orange3,marinkaz/orange3