[libkwineffects] Restore GL_DRAW_FRAMEBUFFER binding in GLTexture::clear

Summary:
If an effect renders a window into an offscreen texture, it's very important
that the window ends up in the offscreen render target rather than the default
framebuffer object. However, that might be not the case if the OpenGL
decoration renderer needs to create a texture atlas since the renderer calls
GLTexture::clear() method, which might clobber the current GL_DRAW_FRAMEBUFFER
binding.

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D25365