DrawTool add drawprepare event

[deyihu]

fix #2074

[codecov-commenter]

Codecov Report

Merging #2075 (5b65159) into master (9db0e54) will decrease coverage by 0.08%.
Report is 2 commits behind head on master.
The diff coverage is 100.00%.

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@@            Coverage Diff             @@
##           master    #2075      +/-   ##
==========================================
- Coverage   83.94%   83.87%   -0.08%     
==========================================
  Files         164      164              
  Lines       18085    18086       +1     
==========================================
- Hits        15181    15169      -12     
- Misses       2904     2917      +13     

Files Changed	Coverage Δ
src/map/tool/DrawTool.js	78.57% <100.00%> (+0.07%)	⬆️

... and 2 files with indirect coverage changes

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