Only use backbuffer mipmaps in SCREEN_TEXTURE when generated. #336

Job	Run time
📊 Static checks / Code style, file formatting, and docs	1m 15s
🍎 macOS / Editor (target=editor, tests=yes)	28m 12s
🤖 Android / Editor (target=editor)	25m 16s
🐧 Linux / Editor w/ Mono (target=editor)	38m 59s
🍎 macOS / Template (target=template_release)	20m 58s
🤖 Android / Template arm32 (target=template_release, arch=arm32)	16m 42s
🍏 iOS / Template (target=template_release)	10m 21s
🐧 Linux / Editor with doubles and GCC sanitizers (target=editor, tests=yes, dev_build=yes, scu_build=yes, precision=double, use_asan=yes, use_ubsan=yes, linker=gold)	45m 54s
🤖 Android / Template arm64 (target=template_release, arch=arm64)	17m 6s
🐧 Linux / Editor with clang sanitizers (target=editor, tests=yes, dev_build=yes, use_asan=yes, use_ubsan=yes, use_llvm=yes, linker=lld)	35m 54s
🐧 Linux / Editor with ThreadSanitizer (target=editor, tests=yes, dev_build=yes, use_tsan=yes, use_llvm=yes, linker=lld)	32m 3s
🌐 Web / Template w/ threads (target=template_release, threads=yes)	25m 18s
🐧 Linux / Template w/ Mono (target=template_release)	27m 42s
🌐 Web / Template w/o threads (target=template_release, threads=no)	25m 50s
🏁 Windows / Editor (target=editor, tests=yes)	50m 11s
🐧 Linux / Minimal template (target=template_release, everything disabled)	15m 37s
🏁 Windows / Editor w/ clang-cl (target=editor, tests=yes, use_llvm=yes)	45m 47s
🏁 Windows / Template (target=template_release)	36m 40s
🪲 Godot CPP / Build and test Godot CPP	8m 20s
	8h 28m 5s

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