QT Changes:
https://codereview.qt-project.org/#/c/166202
https://bugreports.qt.io/browse/QTBUG-33535

This change allows a user to paint on a QOpenGLWidget with a QPainter,
whenever an OpenGL3+ Core Profile format is used or enforced (such as
on macOS).

• Core Profile shaders add that use modern keywords (in, out, etc.).
• Paint engine binds a single VAO and two VBOs.
• Vertex and texture data is uploaded to VBO instead of directly to the GPU.

Krita Changes:

• Request OpenGL 3.2 Core Profile
• Draw our canvas using VAO's if possible
• Homogenised shaders with appropriate versions
• Shader loading happens outside of canvas
• Duplicate shader loading code removed
• Rename supportsGLSL13
• Added Qt patch for ext_qt, see above