Fix T57455: Laggy, freezing UI with Linux and Intel UHD 620

Seems like a driver bug but doing glFlush() before these calls fixes it.

source/blender/gpu/GPU_extensions.h

@@ -66,8 +66,9 @@ typedef enum GPUDeviceType {
 	GPU_DEVICE_NVIDIA =     (1 << 0),
 	GPU_DEVICE_ATI =        (1 << 1),
 	GPU_DEVICE_INTEL =      (1 << 2),
-	GPU_DEVICE_SOFTWARE =   (1 << 3),
-	GPU_DEVICE_UNKNOWN =    (1 << 4),
+	GPU_DEVICE_INTEL_UHD =  (1 << 3),
+	GPU_DEVICE_SOFTWARE =   (1 << 4),
+	GPU_DEVICE_UNKNOWN =    (1 << 5),
 	GPU_DEVICE_ANY =        (0xff)
 } GPUDeviceType;