Created OpenGL Considerations (markdown)

OpenGL-Considerations.md

@@ -0,0 +1,40 @@
+## GL Versions
+
+**GL Version 3.2 is forced**
+
+Because of the limited OpenGL support of macOS, we change the ingame target GL version to 3.2; this includes the GLSL shader version (which is 1.50)
+
+The modules that creates `Lwjgl3ApplicationConfiguration` must add following option to the app config:
+
+```
+appConfig = new Lwjgl3ApplicationConfiguration();
+appConfig.setOpenGLEmulation(Lwjgl3ApplicationConfiguration.GLEmulation.GL30, 3, 2);
+...
+```
+
+### GLSL Versions
+
+**GLSL Version 1.50 is forced**
+
+Every GLSL code must start with following line:
+
+```
+#version 150
+...
+```
+
+and as this change brings significant syntax change, following replacement conventions also apply:
+
+**Vertex Shaders**
+
+| old | new |
+|----|----|
+|`attribute <type> <identifier>`|`in <type> <identifier>`|
+|`varying <type> <identifier>`|`out <type> <identifier>`|
+
+**Fragment Shaders**
+
+| old | new |
+|----|----|
+|`varying <type> <identifier>`|`in <type> <identifier>`|
+|`gl_FragColor = ...`|`out vec4 fragColor;`<br>`fragColor = ...`|