Steps to reproduce:
1. Open attached project "screenshot.zip"
2. Open "test" scene
3. Set Game resolution to "5120x2880" ("res1.png" attached)
4. Run the scene
5. Press spacebar
6. Locate "screenshot.png" in project folder
7. Notice how image resolution doesn't match with the Game resolution ("res2.png" attached)

Note: Works as expected with smaller resolutions. Doesn't reproduce on Windows.

Tested on few Windows computers (dx9/dx11) - no repro
Tested on few iMac's (OpenGL 3.3) - no repro
Tested on few MacBook Pro's (OpenGL 3.3) - successful reproduction (using GeForce GT 320M/GeForce GT 650M)

Reproduced with: 5.3.5p4, 5.4.0b24