improve performance of GC handling

[westram]

Using many GCs has critical performance issues:

• memory:
  • uses ~ 5.4 kb per color
    ⇒ using 64k-colors wastes ~ 350 Mb
    • ~ 250 Mb used by ARB (~ 3.8 kb per color)
    • ~ 100 Mb used by X server (memory is shared between multiple instances of ARB)
  • details in ​spreadsheet
• runtime:
  • slows down startup (initialization of GCs)
  • slows down -refresh (because all GCs are reset to default before show)

Todo:

• check why AW_common::reset_style is used/needed (getting rid of it would be superb).
• AW_GC stores much data about font dimensions
  • this information is font/size-specific
    • ⇒ should not be stored in directly GC
    • instead GC should only reference this data
  • witdh, ascent + descent alone use ~ 1.5 kb (which is ~40% of memory problem)
• Each canvas has its own AW_gc_manager, but for several canvases these are completely identical (e.g. for multiple - or genome-displays). Currently these managers synchronize by using the same awars. They could share alls GCs etc.

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• related to #682
• (maybe) see also comment:5:ticket:703 (AW_common related)

[epruesse]

How much does it cost in motif to change the color of a GC? (In GTK, its nothing but a variable assignment)