internal/graphicscommand: bug fix: flush image buffers
internal/graphicscommand: bug fix: flush image buffers

This is basically a revert for 4d5b608d0cd498e592aadec60d40352d53f09e87.
This caused a crash on Windows.

Even after this fix, the browser tests seem no longer flaky, but
I am not 100% sure.

Updates #2391
Closes #2423

hajimehoshi/ebiten,hajimehoshi/ebiten