Drop glClear() to preserve pixel buffer

According to the OpenGL ES specification, using the discard keyword
should not update the buffer. So glClear() needs to be dropped because
it clears the buffer between renders.

ShaderEditor/src/main/java/de/markusfisch/android/shadereditor/opengl/ShaderRenderer.java

@@ -293,17 +293,18 @@ public void onDrawFrame( GL10 gl )
 				(float)level/scale );
 		}
 
+		if( fb[0] == 0 )
+			createTargets(
+				(int)resolution[0],
+				(int)resolution[1] );
+
+		// first draw custom shader in framebuffer
 		GLES20.glViewport(
 			0,
 			0,
 			(int)resolution[0],
 			(int)resolution[1] );
 
-		if( fb[0] == 0 )
-			createTargets(
-				(int)resolution[0],
-				(int)resolution[1] );
-
 		if( backBufferLoc > -1 )
 		{
 			GLES20.glUniform1i(
@@ -322,13 +323,12 @@ public void onDrawFrame( GL10 gl )
 			GLES20.GL_FRAMEBUFFER,
 			fb[frontTarget] );
 
-		GLES20.glClear(
-			GLES20.GL_COLOR_BUFFER_BIT );
 		GLES20.glDrawArrays(
 			GLES20.GL_TRIANGLE_STRIP,
 			0,
 			4 );
 
+		// then draw framebuffer on screen
 		GLES20.glBindFramebuffer(
 			GLES20.GL_FRAMEBUFFER,
 			0 );
@@ -664,8 +664,8 @@ private void createTargets( int width, int height )
 		createTarget( frontTarget, width, height );
 		createTarget( backTarget, width, height );
 
+		// unbind textures that were bound in createTarget()
 		GLES20.glBindTexture( GLES20.GL_TEXTURE_2D, 0 );
-		GLES20.glBindFramebuffer( GLES20.GL_FRAMEBUFFER, 0 );
 	}
 
 	private void createTarget( int idx, int width, int height )