test/pkg_u8g2: increase main stacksize for SDL

pkg/u8g2/doc.txt

@@ -3,4 +3,16 @@
  * @ingroup  pkg
  * @brief    Provides a monochrome graphics library for OLED and LCD displays
  * @see      https://github.com/olikraus/u8g2
- */
+ *
+ * ## Requirements
+ *
+ * Using SDL requires more stack so in case you are using it add
+ *
+ * ```makefile
+ * CFLAGS += '-DTHREAD_STACKSIZE_MAIN= 48*1024'
+ * ```
+ *
+ * to your makefile.
+ * 48kB is enough for the test other uses may need more or may
+ * need this to be applied to other threads using `THREAD_STACKSIZE_DEFAULT`
+ */

tests/pkg_u8g2/Makefile

@@ -30,6 +30,8 @@ ifeq ($(TEST_OUTPUT),1)
 endif
 
 ifeq ($(TEST_OUTPUT),2)
+# SDL requires more stack
+  CFLAGS += -DTHREAD_STACKSIZE_MAIN=48*1024
   USEMODULE += u8g2_sdl
 endif