Make default `blend_left` consider current blend amount #19226

Job	Run time
📊 Static checks / Code style, file formatting, and docs	34s
🤖 Android / Template (target=template_release)	4m 24s
🍏 iOS / Template (target=template_release)	4m 0s
🍎 macOS / Editor (target=editor, tests=yes)	8m 58s
🌐 Web / Template (target=template_release)	1m 57s
🍎 macOS / Template (target=template_release)	6m 33s
🐧 Linux / Editor w/ Mono (target=editor)	6m 47s
🐧 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)	17m 26s
🏁 Windows / Editor (target=editor, tests=yes)	9m 33s
🐧 Linux / Editor with clang sanitizers (target=editor, tests=yes, dev_build=yes, use_asan=yes, use_ubsan=yes, use_llvm=yes, linker=lld)	8m 17s
🏁 Windows / Template (target=template_release)	7m 46s
🐧 Linux / Editor with ThreadSanitizer (target=editor, tests=yes, dev_build=yes, use_tsan=yes, use_llvm=yes, linker=lld)	7m 33s
🐧 Linux / Template w/ Mono (target=template_release)	2m 0s
🐧 Linux / Minimal template (target=template_release, everything disabled)	1m 30s
🪲 Godot CPP / Build and test Godot CPP	6m 53s
	1h 34m 11s

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