[afterimage] prevent nullptr access (#21860)

Fixes crash reported in https://root-forum.cern.ch/t/timage-drawcircle-cause-core-dump/64784

Provide InitImage method which initialize image for painting
Use it in all relevant places.

Co-authored-by: ferdymercury <ferdymercury@users.noreply.github.com>

Author: ferdymercury

graf2d/asimage/inc/TASImage.h

@@ -77,6 +77,7 @@ class TASImage : public TImage {
    void MapQuality(EImageQuality &quality, UInt_t &asquality, Bool_t toas = kTRUE);
 
    static Bool_t InitVisual();
+   Bool_t InitImage(const char *caller);
 
 public:
    TASImage();