renderer/fb: dont forget to set m_drmFormat (#12833)

fab3370 accidently removed the setting of m_drmFormat, causing it to
think format changed on each alloc.

src/render/Framebuffer.cpp

@@ -52,7 +52,8 @@ bool CFramebuffer::alloc(int w, int h, uint32_t drmFormat) {
     glBindTexture(GL_TEXTURE_2D, 0);
     glBindFramebuffer(GL_FRAMEBUFFER, 0);
 
-    m_size = Vector2D(w, h);
+    m_drmFormat = drmFormat;
+    m_size      = Vector2D(w, h);
 
     return true;
 }