Currently, the invert effect doesn't work because it can't load its
fragment shader because builtin effects are static libs. We need
Q_INIT_RESOURCE() before reading shader code.

This modularizes builtin effects more, which makes easier to add and
remove builtin effects, as well as allows to have per effect resources.

Technically, changing the inner workings of the ShaderManager is an
API incompatible change, but ShaderManager::generateShaderFromResources()
can be used only by builtin effects so it's okay.

ShaderManager::generateShaderFromResources() had to be changed because
two resource files can't share the same prefix. Appending "_core" was
inspired by QtQuick.