[Flutter GPU] Validate render targets, buffer binds and scissors in Dart - #194165

Several invalid inputs went from RenderPass straight to the backends, where the checks are debug-only validation logs or missing, so in a release build they are undefined behavior: an attachment created without render target usage, attachments with different sample counts, a color format as the depth-stencil attachment or a depth-stencil format as a color one, a non-zero mip level on a backend that can't render into one, a BufferView that runs past its buffer, drawIndexed with more indices than the bound index buffer holds, blend state for an attachment that doesn't exist, a scissor outside the render target, and a render target with no attachments at all, which crashes Vulkan. Under the Metal API validation layer the usage and scissor cases abort the process.

All of these are checked in render_pass.dart now and throw before the call reaches native code. The resolve-texture and transient-load checks that already existed ran only inside an assert; _validateAttachments calls them unconditionally, so they hold in release too. An out-of-target scissor throws rather than being clamped, since it is a bug in the caller like every other case here. I looked at the viewport as well: one larger than the target draws fine on all three backends and passes the Metal debug layer, so I left it alone.

Code that passes one of these inputs today and happens to render on its backend will now get an exception. I think that's right for undefined behavior, but it is a change in behavior, and the checks are independent of each other, so I can split this up if that's easier to review. It doesn't overlap #192535 and #192536, the C++ Vulkan case of a target with no color attachment: a depth-only target is still allowed here.

Where it was verified. macOS arm64, host_debug_unopt_arm64, rebased on current main, with one gpu_test.dart test per check (11). Without the change 9 fail on Metal, 10 under flutter_tester_opengles (the mip-level case only applies there), Vulkan on SwiftShader crashes with SIGSEGV on the empty render target, and Metal with MTL_DEBUG_LAYER=1 aborts on the usage case. With it all of them pass on the three backends, Metal under the debug layer included. The resolve-texture test passes either way under flutter_tester, which always runs Dart with asserts on, so it can't show the release gap; it pins the messages.

Fixes #194155

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the AI contribution guidelines and understand my responsibilities, or I am not using AI tools.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant in-code documentation (doc comments with ///).
• If this PR introduces a new feature or capability, I created and linked a website documentation issue or PR in flutter/website (or verified none is needed).
• I added new tests to check the change I am making, or this PR is test-exempt.
• I followed the breaking change policy and added Data Driven Fixes where supported.
• All existing and new tests are passing.