Warning fixes (fix #118)

imgui.h

@@ -704,7 +704,7 @@ struct ImColor
 {
     ImVec4              Value;
 
-    ImColor(int r, int g, int b, int a = 255)                       { Value.x = r / 255.0f; Value.y = g / 255.0f; Value.z = b / 255.0f; Value.w = a / 255.0f; }
+    ImColor(int r, int g, int b, int a = 255)                       { Value.x = (float)r / 255.0f; Value.y = (float)g / 255.0f; Value.z = (float)b / 255.0f; Value.w = (float)a / 255.0f; }
     ImColor(float r, float g, float b, float a = 1.0f)              { Value.x = r; Value.y = g; Value.z = b; Value.w = a; }
     ImColor(const ImVec4& col)                                      { Value = col; }