[gpad] avoid potential float precision lost in PaintHatches

[linev]

Use integer conversion once and then just check boundary with ymin

Alternative to #18494

[ferdymercury]

Side note: an integer for loop is faster than doing a double comparison on every loop (while) step for stopping. Can be optimized better by compiler, too. That's why I suggested precalculating the number of steps. The number of steps is a very determined value, no need to leave the runtime double comparison that task in my opinion.

[linev]

@ferdymercury

There are several other potential problems:

• dy can be 0 after gStyle->SetHatchesSpacing(0), negative values also will be problematic
• vertical hatches not handled but emulated with angle 89.99 making artifact with fill style 3409 from core/base/doc/macros/fillpatterns.C macros
  

I do not think that few more double operations are real problem here.

[linev]

void SetHatchesSpacing(Double_t h) {fHatchesSpacing = TMath::Max(0.1,h);}

There is protection.

[github-actions]

Test Results

    18 files      18 suites   4d 1h 33m 46s ⏱️
 2 731 tests  2 730 ✅ 0 💤 1 ❌
47 719 runs  47 718 ✅ 0 💤 1 ❌

For more details on these failures, see this check.

Results for commit 0d2f8c4.