Optimization and features added

[serhack]

The PR was created for:

• managing a custom hex color passed by console via the -color flag Specify the background color as a flag #2
• using alpha compositing for creating the background color without weird rules Make background color more transparent #1
• achieving better execution time (still I/O bottleneck)

The PR contains the follow optimizations:

• move the matrix to heap (thanks to make)
• adding a flag that specifies if a pixel has already been modified
• edit float64 to uint8 to reduce memory occupation