Backends: Glfw, SDL2: ImplXXXGetContentScale shall return 1 on emscripten / apple / android

This is a fix related to #8733

We can divide platforms into two cases based on how they report screen geometry:

    Case 1: Platforms which report screen size in "physical pixels": Windows (for "Dpi aware" apps), Linux (with Wayland)
    Case 2: Platforms which report screen size in "density-independent pixels": macOS, iOS, Android, emscripten

As a consequence, there are two important things we need to know:

    FramebufferScale: The scaling factor FrameBufferSize / ScreenSize
        In case 1, the framebuffer size is equal to the screen size and DisplayFramebufferScale=1.
        In case 2, the framebuffer size is equal to the screen size multiplied by a factor, for example DisplayFramebufferScale=2.
    ContentScale The scaling factor for the content that we will display
        In case 1, the content scale will often need to be > 1 (e.g., 2), because we will need to display bigger elements so that they show with a correct physical size on the screen.
        In case 2, the content scale is equal to 1

This commit fixes ContentScale for platforms in case 2.

Author: pthom

backends/imgui_impl_glfw.cpp

@@ -876,7 +876,7 @@ static void ImGui_ImplGlfw_UpdateGamepads()
 // - Some accessibility applications are declaring virtual monitors with a DPI of 0.0f, see #7902. We preserve this value for caller to handle.
 float ImGui_ImplGlfw_GetContentScaleForWindow(GLFWwindow* window)
 {
-#if GLFW_HAS_PER_MONITOR_DPI && !defined(__APPLE__)
+#if GLFW_HAS_PER_MONITOR_DPI && !(defined(__APPLE__) || defined(__EMSCRIPTEN__) || defined(__ANDROID__))
     float x_scale, y_scale;
     glfwGetWindowContentScale(window, &x_scale, &y_scale);
     return x_scale;
@@ -888,7 +888,7 @@ float ImGui_ImplGlfw_GetContentScaleForWindow(GLFWwindow* window)
 
 float ImGui_ImplGlfw_GetContentScaleForMonitor(GLFWmonitor* monitor)
 {
-#if GLFW_HAS_PER_MONITOR_DPI && !defined(__APPLE__)
+#if GLFW_HAS_PER_MONITOR_DPI && !(defined(__APPLE__) || defined(__EMSCRIPTEN__) || defined(__ANDROID__))
     float x_scale, y_scale;
     glfwGetMonitorContentScale(monitor, &x_scale, &y_scale);
     return x_scale;

backends/imgui_impl_sdl2.cpp

@@ -718,7 +718,7 @@ float ImGui_ImplSDL2_GetContentScaleForWindow(SDL_Window* window)
 float ImGui_ImplSDL2_GetContentScaleForDisplay(int display_index)
 {
 #if SDL_HAS_PER_MONITOR_DPI
-#ifndef __APPLE__
+#if !defined(__APPLE__) && !defined(__EMSCRIPTEN__) && !defined(__ANDROID__)
     float dpi = 0.0f;
     if (SDL_GetDisplayDPI(display_index, &dpi, nullptr, nullptr) == 0)
         return dpi / 96.0f;