Summary:
The streaming buffer is shared so we have to reset it before rendering
something. Otherwise, it's basically undefined behavior.

For example, if the streaming buffer had been used for rendering a
geometry with some color, then the Background Contrast effect may crash
in GLVertexBuffer::bindArrays, it all depends on whether there is any
bounded GLShader. To fix that, the buffer has to be reset.

BUG: 356141

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: davidedmundson, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D19034