D3D12: Avoid cases of redundant render target clears #847

Job	Run time
📊 Static checks / Code style, file formatting, and docs	1m 1s
🍎 macOS / Editor (target=editor, tests=yes)	30m 32s
🤖 Android / Editor (target=editor)	25m 3s
🍎 macOS / Template (target=template_release)	22m 5s
🍏 iOS / Template (target=template_release)	10m 36s
🤖 Android / Template arm32 (target=template_release, arch=arm32)	16m 39s
🐧 Linux / Editor w/ Mono (target=editor)	40m 37s
🤖 Android / Template arm64 (target=template_release, arch=arm64)	16m 56s
🐧 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)	47m 38s
🌐 Web / Template w/ threads (target=template_release, threads=yes)	25m 57s
🐧 Linux / Editor with clang sanitizers (target=editor, tests=yes, dev_build=yes, use_asan=yes, use_ubsan=yes, use_llvm=yes, linker=lld)	36m 34s
🏁 Windows / Editor (target=editor, tests=yes)	50m 39s
🐧 Linux / Editor with ThreadSanitizer (target=editor, tests=yes, dev_build=yes, use_tsan=yes, use_llvm=yes, linker=lld)	35m 0s
🌐 Web / Template w/o threads (target=template_release, threads=no)	26m 9s
🏁 Windows / Template (target=template_release)	34m 27s
🐧 Linux / Template w/ Mono (target=template_release)	27m 11s
🐧 Linux / Minimal template (target=template_release, everything disabled)	15m 54s
🪲 Godot CPP / Build and test Godot CPP	7m 49s
	7h 50m 47s

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