Populate jaggl_alpha_bits with cAlphaBits from selected pixel format

gl-natives/src/main/c/jaggl.c

@@ -521,7 +521,11 @@ JNIEXPORT jboolean JNICALL Java_jaggl_context_choosePixelFormat1(JNIEnv *env, jc
 		goto dsi_free;
 	}
 
-	// TODO(gpe): get actual number of alpha bits
+	if (!DescribePixelFormat(jaggl_device, format, sizeof(pfd), &pfd)) {
+		goto dsi_free;
+	}
+
+	jaggl_alpha_bits = pfd.cAlphaBits;
 
 	result = JNI_TRUE;
 #else