Implement 2D graphic API (#87)

* Implement 2D graphic library based on LittlevGL

* Add lvgl license file

samples/gui/wasm-apps/wgl/Makefile

@@ -0,0 +1,39 @@
+# Copyright (C) 2019 Intel Corporation.  All rights reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+CC = emcc
+APP_DIR = ${shell pwd}
+IWASM_DIR=../../../../core/iwasm
+CFLAGS += -O3 \
+	  -Wno-int-conversion \
+	  -DLV_CONF_INCLUDE_SIMPLE \
+	  -I$(APP_DIR)/src/ \
+	  -I$(IWASM_DIR)/lib/app-libs/base/ \
+	  -I$(IWASM_DIR)/lib/native-interface/ \
+	  -I$(IWASM_DIR)/lib/app-libs/extension/sensor \
+	  -I$(IWASM_DIR)/lib/app-libs/extension/gui \
+	  -I$(IWASM_DIR)/lib/app-libs/extension/connection
+
+SRCS += $(APP_DIR)/src/main.c
+
+# For app size consideration, not all but necessary app libs are included
+SRCS += $(IWASM_DIR)/lib/app-libs/base/timer.c
+SRCS += $(IWASM_DIR)/lib/app-libs/extension/gui/src/*.c
+
+all: 
+	@$(CC) $(CFLAGS) $(SRCS) \
+    -s WASM=1 -s SIDE_MODULE=1 -s ASSERTIONS=1 -s STACK_OVERFLOW_CHECK=2 \
+    -s TOTAL_MEMORY=65536 -s TOTAL_STACK=2048\
+    -s "EXPORTED_FUNCTIONS=['_on_init', '_on_timer_callback', '_on_widget_event']" \
+    -o ui_app.wasm