Spurious return characters

• Build tested: sdl2 branch commit 0e95d118
• Tested on: macOS Catalina 10.15.5 and Ubuntu 20.04
• Tested sdl2trs configuration: Model III 48K disk system with Radio Shack hi-res

I've noticed sdl2trs sometimes generates spurious return characters, as if someone was constantly pressing the Enter key.

I've found one way to consistently replicate this problem:

• Boot Model III with standard Radio Shack hi-res boot disk (see hiresa.dmk attached) and enter date/time.
• At TRSDOS Ready prompt, enter "GLOAD GPICTURE"
• After the hi-res image has been loaded and it returns to text mode, there will be a pause of about a second, and then you'll see the spurious return characters, i.e. the TRSDOS Ready prompt will keep appearing and will only stop when you start pressing keys.

I've tested this same disk image on the TRS32 emulator (under Windows) and on a real Model III, and it doesn't have the same problem.

Interestingly the issue doesn't occur if I set the Display Setting "Resize Window on Mode Change for Model 3" to "No" (the default is "Yes"). I.e. there's possibly a problem in the mode change code.

hiresa.dmk