Fix crash when executing SubViewport.set_size_2d_override_stretch

[timothyqiu]

Fixes #65456

I did not find a way to reproduce the crash, but the cause of the crash is quite obvious according to the crash log.

• sample_count is a VkSampleCountFlagBits enum, it's equal to the actual number of bits (1, 2, 4, 8, 16, ...).
• rasterization_sample_count is an array to map TextureSamples to VkSampleCountFlagBits.
• p_requested_sample_count is a TextureSamples enum, it can be used to index rasterization_sample_count.

So using sample_count to index rasterization_sample_count will result in a crash when it's greater than TEXTURE_SAMPLES_MAX (7).

sample_count should be used directly like in the other if branch.

[akien-mga]

Thanks!