integrate and link lib curl for android, add VAO support for brush draw, sync canvas with screen redraw instead of input events

engine/font.cpp

@@ -16,7 +16,11 @@ bool Font::load(const char* ttf, int font_size)
         auto bitmap = std::make_unique<uint8_t[]>(w*h);
         chars.resize(num_chars);
         int ret = stbtt_BakeFontBitmap(file.m_data, 0, (float)font_size, bitmap.get(), w, h, start_char, num_chars, chars.data());
+#ifdef __APPLE__
         font_tex.create(w, h, GL_RED, bitmap.get());
+#else
+        font_tex.create(w, h, GL_LUMINANCE, bitmap.get());
+#endif // __APPLE__
         file.close();
         return true;
     }