Added workaround for missing shader version numbers

If the program cannot be compiled, a version number
is added to the code, and the compilation is started
again. This is very pragmatic, and should not be
necessary when shaders are versioned properly.

jgltf-viewer-jogl/src/main/java/de/javagl/jgltf/viewer/jogl/GlContextJogl.java

@@ -89,8 +89,8 @@ public Integer createGlProgram(
     {
         logger.fine("Creating vertex shader...");
 
-        Integer glVertexShader = 
-            createGlShader(GL_VERTEX_SHADER, vertexShaderSource);
+        Integer glVertexShader = createGlShader(
+            GL_VERTEX_SHADER, vertexShaderSource);
         if (glVertexShader == null)
         {
             logger.warning("Creating vertex shader FAILED");
@@ -100,8 +100,8 @@ public Integer createGlProgram(
         logger.fine("Creating vertex shader DONE");
 
         logger.fine("Creating fragment shader...");
-        Integer glFragmentShader = 
-            createGlShader(GL_FRAGMENT_SHADER, fragmentShaderSource);
+        Integer glFragmentShader = createGlShader(
+            GL_FRAGMENT_SHADER, fragmentShaderSource);
         if (glFragmentShader == null)
         {
             logger.warning("Creating fragment shader FAILED");
@@ -182,6 +182,39 @@ public void disable(Iterable<? extends Number> states)
      * @return The GL shader
      */
     private Integer createGlShader(int shaderType, String shaderSource)
+    {
+        Integer glShader = createGlShaderImpl(shaderType, shaderSource);
+        if (glShader != null)
+        {
+            return glShader;
+        }
+
+        // If the shader source code does not contain a #version number, 
+        // then, depending on the com.jogamp.opengl.GLProfile that was 
+        // chosen for the viewer, certain warnings may be treated as 
+        // errors. As a workaround, pragmatically insert a version 
+        // number and try again...
+        // (Also see https://github.com/javagl/JglTF/issues/12)
+        if (!shaderSource.contains("#version"))
+        {
+            String versionString = "#version 120";
+            logger.warning("Inserting GLSL version specifier \"" + 
+                versionString + "\" in shader code");
+            String shaderSourceWithVersion = 
+                versionString + "\n" + shaderSource;
+            return createGlShaderImpl(shaderType, shaderSourceWithVersion);
+        }
+        return null;
+    }
+
+    /**
+     * Implementation for {@link #createGlShader(int, String)}.
+     * 
+     * @param shaderType The shader type
+     * @param shaderSource The shader source code
+     * @return The GL shader, or <code>null</code> if it cannot be compiled
+     */
+    private Integer createGlShaderImpl(int shaderType, String shaderSource)
     {
         int glShader = gl.glCreateShader(shaderType);
         gl.glShaderSource(
@@ -195,8 +228,10 @@ private Integer createGlShader(int shaderType, String shaderSource)
             return null;
         }
         return glShader;
+
     }
 
+
     @Override
     public int getUniformLocation(int glProgram, String uniformName)
     {