Handle transparency

[furti]

The import does not handle transparency right now.

• Transparency should be handled as "white" by default
• Make it possible to change the "height" for the alpha channel

[furti]

For now the implementation will be as follows:

• If the alpha value for a pixel is lower than 255 we use the alpha channel to calculate the height
• 254 means black and 0 means white (the more transparent the lower the resulting geometry)