vulkan_device: keep an integrated GPU's budget within its memory share

CollectPhysicalMemoryInfo gives an integrated GPU 4 GiB of device memory
(6 GiB with aggressive VRAM use), since its device-local heap is system
memory. GetDeviceMemoryBudget returned the heap's driver budget instead
(21178 MiB on a Radeon 780M), and the texture cache follows that budget
for Switch 2 titles: it collected only past 20.6 GB, filling system RAM,
and kept ASTC uncompressed (the BC3 switch looks at the budget too). Cap
it at that share.

Radeon 780M, Mario Kart World title screen, 200 s: shared GPU memory
2.7 -> 3.6 GB instead of 7.6 -> 12.7 GB (0.25 instead of 1.8 GB/min),
private bytes 12.9 instead of 22.4 GB at the end, ASTC re-compressed to
BC3, same frame rate.

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