Solve a set of bugs on cameraInit

[servantftechnicolor]

Updates to camera init :

• when no information available, sensorwidth was set to 36 and sensorHeight was set to 24 not considering the image ratio
• The effect of pixel aspect ratio on "focal length" (scale) is updated
• The default offset is assigned even if no valid focal length is known
• Initial focal length was enforced during sfm even if the sensor width was only guessed