Create dedicated enum for EFB/XFB gamma correction

This also changes the behavior for the invalid gamma value, which was confirmed to behave the same as 2.2.

Note that currently, the gamma value is only used for XFB copies, even though hardware testing indicates it also works for EFB copies.  This will be changed in a later commit.

Source/Core/Core/FifoPlayer/FifoPlayer.cpp

@@ -593,7 +593,7 @@ void FifoPlayer::ClearEfb()
   copy.clamp_bottom = false;
   copy.unknown_bit = false;
   copy.target_pixel_format = static_cast<u32>(EFBCopyFormat::RGBA8) << 1;
-  copy.gamma = 0;
+  copy.gamma = GammaCorrection::Gamma1_0;
   copy.half_scale = false;
   copy.scale_invert = false;
   copy.clear = true;