[lookandfeel] Fix GLES incompatibilities in UserDelegate shader code

Summary:
BUG: 382668

The shader compilation currently fails on GLES with errors like:
"0:6(2): error: No precision specified in this scope for type `vec4'"

GLES requires variable qualifiers like highp/lowp, whereas desktop OpenGL
does not. As QGlShaderProgram adds suitable defines on desktop OpenGL
for these qualifiers, it is safe to add these to a declarations, see:
http://doc.qt.io/qt-5/qglshaderprogram.html#writing-portable-shaders

Reviewers: #plasma, davidedmundson

Reviewed By: #plasma, davidedmundson

Subscribers: plasma-devel

Tags: #plasma

Maniphest Tasks: T7071

Differential Revision: https://phabricator.kde.org/D7770