Fix color scheme with gfx

- gfx uses an sRGB framebuffer, which means it expects all vertex colors to be in linear space
- imgui provides vertex colors in sRGB space! This causes the appearance of washed out colors
- Fix the color space conflict by converting the imgui colors to linear space

imgui-examples/examples/hello_gfx.rs

@@ -4,6 +4,7 @@ extern crate glutin;
 #[macro_use]
 extern crate imgui;
 extern crate imgui_gfx_renderer;
+extern crate imgui_sys;
 
 use imgui::*;
 

imgui-examples/examples/support_gfx/mod.rs

@@ -43,6 +43,23 @@ pub fn run<F: FnMut(&Ui) -> bool>(title: String, clear_color: [f32; 4], mut run_
     };
 
     let mut imgui = ImGui::init();
+    {
+        // Fix incorrect colors with sRGB framebuffer
+        use imgui_sys::ImVec4;
+
+        fn imgui_gamma_to_linear(col: ImVec4) -> ImVec4 {
+            let x = col.x.powf(2.2);
+            let y = col.y.powf(2.2);
+            let z = col.z.powf(2.2);
+            let w = 1.0 - (1.0 - col.w).powf(2.2);
+            ImVec4::new(x, y, z, w)
+        }
+
+        let style = imgui.style_mut();
+        for col in 0..style.colors.len() {
+            style.colors[col] = imgui_gamma_to_linear(style.colors[col]);
+        }
+    }
     imgui.set_ini_filename(None);
     let config = ImFontConfig::new().oversample_h(1).pixel_snap_h(true).size_pixels(13.0);
     config.rasterizer_multiply(1.75).add_font(