direct: Use explicitly Vec3F in calls to addData3f to avoid crash on double precision builds

[el-dee]

Issue description

When opening the BufferViewer with Panda3D compiled with double precision float enabled, the app crashes as Vec3 is actually Vec3D and addData3f() expect Vec3F

Solution description

Trivial fix that replace Vec3 with Vec3F

Checklist

I have done my best to ensure that…

• …I have familiarized myself with the CONTRIBUTING.md file
• …this change follows the coding style and design patterns of the codebase
• …I own the intellectual property rights to this code
• …the intent of this change is clearly explained
• …existing uses of the Panda3D API are not broken
• …the changed code is adequately covered by the test suite, where possible.

[codecov]

Codecov Report

Merging #1365 (e809f57) into master (d239f48) will not change coverage.
The diff coverage is 0.00%.

@@           Coverage Diff           @@
##           master    #1365   +/-   ##
=======================================
  Coverage   16.10%   16.10%           
=======================================
  Files        3740     3740           
  Lines      363744   363744           
=======================================
  Hits        58594    58594           
  Misses     305150   305150           

Impacted Files	Coverage Δ
direct/src/showbase/BufferViewer.py	20.57% <0.00%> (ø)

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[rdb]

Oops, the timing is unfortunate, since I had already reached the point of no return for the 1.10.12 release at the time you filed this PR. It will have to go into 1.10.13, with my apologies for the regression.

Thanks!