renderer_opengl: Fix sRGB blits

Removes the sRGB hack of tracking if a frame used an sRGB rendertarget
to apply at least once to blit the final texture as sRGB. Instead of
doing this apply sRGB if the presented image has sRGB.

Also enable sRGB by default on Maxwell3D registers as some games seem to
assume this.

src/video_core/engines/maxwell_3d.cpp

@@ -89,6 +89,9 @@ void Maxwell3D::InitializeRegisterDefaults() {
 
     // Commercial games seem to assume this value is enabled and nouveau sets this value manually.
     regs.rt_separate_frag_data = 1;
+
+    // Some games (like Super Mario Odyssey) assume that SRGB is enabled.
+    regs.framebuffer_srgb = 1;
 }
 
 #define DIRTY_REGS_POS(field_name) (offsetof(Maxwell3D::DirtyRegs, field_name))