opengl: Drop GLTexture::clear()

It's used only by the decoration renderer, but even it doesn't need it
because the atlas parts are padded.

From the API point of view, it's worth looking for alternative solutions,
like integrating the render target clear step in the render passes. And
texture uploading code usually doesn't need to clear the texture because
it is going to overwrite its contents anyway.

src/opengl/gltexture.cpp

@@ -353,17 +353,6 @@ GLenum GLTexture::internalFormat() const
     return d->m_internalFormat;
 }
 
-void GLTexture::clear()
-{
-    Q_ASSERT(d->m_owning);
-    GLFramebuffer fbo(this);
-    GLFramebuffer::pushFramebuffer(&fbo);
-    glClearColor(0, 0, 0, 0);
-    glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, d->m_texture, 0);
-    glClear(GL_COLOR_BUFFER_BIT);
-    GLFramebuffer::popFramebuffer();
-}
-
 bool GLTexture::isDirty() const
 {
     return d->m_markedDirty;

src/opengl/gltexture.h

@@ -98,10 +98,6 @@ public:
 
     QImage toImage();
 
-    /** @short
-     * Make the texture fully transparent
-     */
-    void clear();
     /**
      * @deprecated track modifications to the texture yourself
      */

src/scene/workspacescene_opengl.cpp

@@ -472,7 +472,6 @@ void SceneOpenGLDecorationRenderer::resizeTexture()
         m_texture->setContentTransform(OutputTransform::FlipY);
         m_texture->setFilter(GL_LINEAR);
         m_texture->setWrapMode(GL_CLAMP_TO_EDGE);
-        m_texture->clear();
     } else {
         m_texture.reset();
     }