[cc65] TGI API deficit / proposal

Hi,

when looking the other day at the 3dmaze output I noticed that it is
rather ugly that the screen is always cleared an then redrawed. Then I
remembered that a native Apple2 program would certainly used
offscreen-drawing and page-switching to improve the visual appearance.

But when looking at the TGI API to see if that would be implementable
with TGI I noticed that tgi_clear() always clears the current, visible
screen :-(

From my experience this turns the multi-page support in TGI rather
useless: I see (maybe oversimplifying) three ways to handle the
flicker-free screeen drawing in interactive programs:

1. Modify a single page incrementally (doesn't need multi-page support at all).
2. Redraw the hidden page opaquely (feasable only with bitmaps
unfortunately not supported by TGI at all)
3. Clear hidden page and then draw selectively (needs page-specific
clear currently not available in TGI)

With a page-specific clear the 3dmaze could by easily converted to a
type 3.) program - and by checking tgi_getpagecount() this behaviour
could be turned on only if the driver supports it...

Best, Oliver
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