Implement sRGB support for QSurfaceFormat / QOpenGLWidget

Not having proper sRGB support is a huge problem and makes it impossible for me to render my application correctly.

My current pipeline looks like this:
1. Load textures in sRGB format (GL_SRGB8_ALPHA8)
2. Render textures to GBuffer (GL_RGBA8)
3. Render lights (some with sRGB loaded textures) to an accumulation buffer (GL_RGBA16F), using GBuffer as input
4. Do HDR tonemapping from the accumulation buffer to a composition buffer (GL_SRGB8 - to preserve precision)
5. Render composition buffer to screen

Using a QOpenGLWidget, "screen" in number 5 of course means another framebuffer. I also have GL_FRAMEBUFFER_SRGB enabled in both applications.

Using this exact render pipeline yields accurate results in my non-Qt application (OpenGL Core 4.5 profile, created using GLFW3 with GLFW_SRGB_CAPABLE hint set to true)

Using this exact render pipeline yields dark results in my Qt-application (OpenGL Core 4.5 profile, created using QSurfaceFormat::setDefaultFormat() before instancing application).