Add content scale queries

This adds glfwGetWindowContentScale and glfwGetMonitorContentScale for
querying the recommended drawing scale factor for DPI-aware rendering.

Parts of this patch are based on code by @ferreiradaselva.

Fixes #235.
Fixes #439.
Fixes #677.
Fixes #845.
Fixes #898.

src/win32_platform.h

@@ -135,6 +135,13 @@ typedef enum PROCESS_DPI_AWARENESS
     PROCESS_SYSTEM_DPI_AWARE = 1,
     PROCESS_PER_MONITOR_DPI_AWARE = 2
 } PROCESS_DPI_AWARENESS;
+typedef enum MONITOR_DPI_TYPE
+{
+    MDT_EFFECTIVE_DPI = 0,
+    MDT_ANGULAR_DPI = 1,
+    MDT_RAW_DPI = 2,
+    MDT_DEFAULT = MDT_EFFECTIVE_DPI
+} MONITOR_DPI_TYPE;
 #endif /*DPI_ENUMS_DECLARED*/
 
 // HACK: Define versionhelpers.h functions manually as MinGW lacks the header
@@ -216,7 +223,9 @@ typedef HRESULT(WINAPI * PFN_DwmEnableBlurBehindWindow)(HWND,const DWM_BLURBEHIN
 
 // shcore.dll function pointer typedefs
 typedef HRESULT (WINAPI * PFN_SetProcessDpiAwareness)(PROCESS_DPI_AWARENESS);
+typedef HRESULT (WINAPI * PFN_GetDpiForMonitor)(HMONITOR,MONITOR_DPI_TYPE,UINT*,UINT*);
 #define SetProcessDpiAwareness _glfw.win32.shcore.SetProcessDpiAwareness_
+#define GetDpiForMonitor _glfw.win32.shcore.GetDpiForMonitor_
 
 typedef VkFlags VkWin32SurfaceCreateFlagsKHR;
 
@@ -326,6 +335,7 @@ typedef struct _GLFWlibraryWin32
     struct {
         HINSTANCE                       instance;
         PFN_SetProcessDpiAwareness      SetProcessDpiAwareness_;
+        PFN_GetDpiForMonitor            GetDpiForMonitor_;
     } shcore;
 
 } _GLFWlibraryWin32;
@@ -395,4 +405,5 @@ void _glfwInitTimerWin32(void);
 void _glfwPollMonitorsWin32(void);
 GLFWbool _glfwSetVideoModeWin32(_GLFWmonitor* monitor, const GLFWvidmode* desired);
 void _glfwRestoreVideoModeWin32(_GLFWmonitor* monitor);
+void _glfwGetMonitorContentScaleWin32(HMONITOR handle, float* xscale, float* yscale);