GPU: Implemented the F2F 'round' rounding mode.

It's implemented via the GLSL 'roundEven()' function.

src/video_core/renderer_opengl/gl_shader_decompiler.cpp

@@ -1213,6 +1213,9 @@ private:
                 switch (instr.conversion.f2f.rounding) {
                 case Tegra::Shader::F2fRoundingOp::None:
                     break;
+                case Tegra::Shader::F2fRoundingOp::Round:
+                    op_a = "roundEven(" + op_a + ')';
+                    break;
                 case Tegra::Shader::F2fRoundingOp::Floor:
                     op_a = "floor(" + op_a + ')';
                     break;