consolidate debug graphs for IR into one .png file

[pointbazaar]

Currently, when running e.g.

sd -x86 -debug main.dg

where main.dg has 3 functions:

• main
• f
• g

it will generate 3 files:

• main.png
• f.png
• g.png

with the IR representation (struct TAC) in a graph, visualizing the control flow between basic blocks (struct BasicBlock)
in these functions.

Since most of these test/example programs tend to be rather simple,
it may be better to put all these graphs into one .png instead, making it easier
to reason across functions.

So it should only generate main.png

The basic blocks belonging to each functions can be marked as such, to clarify where they come from.