Fixed compiling on windows after .hdr support

Changed type to avoid casting.

modules/hdr/image_loader_hdr.cpp

@@ -125,7 +125,7 @@ Error ImageLoaderHDR::load_image(Ref<Image> p_image, FileAccess *f) {
 		//convert
 		for (int i = 0; i < width * height; i++) {
 
-			float exp = pow(2, ptr[3] - 128);
+			float exp = pow(2.0f, ptr[3] - 128.0f);
 
 			Color c(
 					ptr[0] * exp / 255.0,