platforms/drm: Properly clean up the shadow framebuffer object

In case hardware transforms can be used again, the shadow framebuffer
must be destroyed; otherwise rendered results will be distorted due to a
mismatch between the dimensions of the shadow framebuffer and the mode
size.

plugins/platforms/drm/egl_gbm_backend.cpp

@@ -345,6 +345,8 @@ const float texCoords[] = {
 
 bool EglGbmBackend::resetFramebuffer(Output &output)
 {
+    cleanupFramebuffer(output);
+
     if (output.output->hardwareTransforms() && !output.onSecondaryGPU) {
         // No need for an extra render target.
         return true;
@@ -352,8 +354,6 @@ bool EglGbmBackend::resetFramebuffer(Output &output)
 
     makeContextCurrent(output);
 
-    cleanupFramebuffer(output);
-
     glGenFramebuffers(1, &output.render.framebuffer);
     glBindFramebuffer(GL_FRAMEBUFFER, output.render.framebuffer);
     GLRenderTarget::setKWinFramebuffer(output.render.framebuffer);