Missing header file note ofxOpenGLRender.h

http://openeffects.org/standard_changes/modifying-plugin-properties-after-description
states that a plugin instance can change the Enable state in Instance Changed with the following precaution advised for plugins that do so:

"
When setting to true we also do this (assuming it's not set):
// we support all the OpenGL bit depths
gPropHost->propSetString(effectProps, kOfxOpenGLPropPixelDepth, 0, kOfxBitDepthByte);
gPropHost->propSetString(effectProps, kOfxOpenGLPropPixelDepth, 1, kOfxBitDepthShort);
gPropHost->propSetString(effectProps, kOfxOpenGLPropPixelDepth, 2, kOfxBitDepthFloat);
gPropHost->propSetString(effectProps, kOfxOpenGLPropPixelDepth, 3, kOfxBitDepthHalf );

"

Start with the missing note requested for ofxOpenGLRender.h and compare it with the linked OpenFX standard change guidance. Document the precaution about setting the OpenGL pixel-depth properties when enabling a plugin instance; done means the relevant header contains that guidance.

Written by the indexing model from the issue text.