[GrafX2] #174: Rebase and merge the old SDL2 branch (accelerated scaling)

[GrafX2]

#174: Rebase and merge the old SDL2 branch (accelerated scaling)
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Reporter: PulkoMandy | Owner: pulkomandy
Type: defect | Status: new
Priority: major | Milestone: 2.8
Component: GrafX2 | Version: 2.9WIP
Keywords: |
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Currently GrafX2 uses software scaling to implement the "double",
"triple", "wide" and "tall" pixels. This dates back from the SDL1 port
(DOS used to tweak videocard parameters directly instead).

This approach works OK for SDL1, but it is CPU hungry. In SDL2 it would
make more sense to use the hardware to do it.

This work was started in the original SDL2 branch:
https://gitlab.com/GrafX2/grafX2/-/tree/sdl2?ref_type=heads but it was not
fully complete, and then an independant migration to SDL2 was done. Then
an additional support for hardware scaling was added in
https://gitlab.com/GrafX2/grafX2/-/merge_requests/362 but it is on top of
the existing scalers, making things more complex and more confusing.

So the plan is:
- Remove that extra layer of scaling
- Rebase the SDL2 branch or rewrite something along these lines, using
hardware scaling to implement double/wide/tall pixels

This has become a bit more complex than it was in DOS, because now we take
advantage of the "native" pixels for some things. For example, the layer
previews (when hovering the layer bar) and the preview in the file
load/save dialog use the full screen resolution
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Ticket URL: <http://pulkomandy.tk/projects/GrafX2/ticket/174>
GrafX2 <http://pulkomandy.tk/projects/GrafX2>
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