Render shadows

[danlipsa]

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[danlipsa]

@mleotta @aashish24 This is the same MR as on gitlab. These are the remaining issues with code:

• The shadow is not fully marked.
• Probably I should generate the shadow masks as a separate file instead of marked as part of a orthorectified image.

See the attached image. I use yellow to mark the shadow. The right image is a DSM, changed into a surface, rendered from the sun position, orthorectified and having the shadows extracted using the orthorectification code. The left image is a PAN image orthorectified.