[gpad] make canvas size consistent in batch vs non-batch

[ferdymercury]

Fixes #11004

Explores alternative solution to #22226

fyi @rlalik

TODO:

• Test if this offset is platform-dependent or not. Maybe we need if MACOS -= 4 else -=2
• Not sure if lines 234 and 880 need to be adapted, too.
• Maybe create ctest

Reproducer:

// Run either as root -l test_pixels.cpp -b -q
// Or as xvfb-run root -l test_pixels.cpp -q
#include <TCanvas.h>
#include <TROOT.h>
#include <iostream>
void test_pixels() {
    for(Int_t w=500;w<=900;w++) {
        for(Int_t h=500;h<=600;h++) {
            TCanvas *c = new TCanvas("c1","c1",w,h);
            Int_t diffw = w-c->GetWw();
            Int_t diffh = h-c->GetWh();
            if(!gROOT->IsBatch()) {
                if (diffw !=2 || diffh !=24)
                    std::cout << diffw << " " << diffh << " " << w << " " << h << std::endl;
            }
            else {
                if (diffw !=2 || diffh !=24)
                    std::cout << diffw << " " << diffh << " " << w << " " << h << std::endl;
            }
            delete c;
        }
    }
}

[github-actions]

Test Results

    22 files      22 suites   3d 11h 5m 31s ⏱️
 3 849 tests  3 844 ✅ 0 💤  5 ❌
76 935 runs  76 852 ✅ 0 💤 83 ❌

For more details on these failures, see this check.

Results for commit b2ec340.