Fix x86 crash: CreateTexture command buffer read uses wrong type (#3643)

The _external parameter is written to the command buffer as uint64_t (8 bytes)
but read back as uintptr_t (4 bytes on 32-bit). This causes a 4-byte
deserialization misalignment on x86, corrupting all subsequent command
buffer reads and leading to EXCEPTION_ACCESS_VIOLATION (0xC0000005) during
D3D11 initialization.

Co-authored-by: Matteo Valdina <mavaldin@microsoft.com>

src/bgfx.cpp

@@ -3344,7 +3344,7 @@ namespace bgfx
 					uint8_t skip;
 					_cmdbuf.read(skip);
 
-					uintptr_t external;
+					uint64_t external;
 					_cmdbuf.read(external);
 
 					void* ptr = m_renderCtx->createTexture(handle, mem, flags, skip, external);