bb75cbc0003033ea73c42ef02370767aa52d6ba6  [CoordinatedGraphics] Move texture mapper specific code to their own files

Classified as not security-relevant.
Triage note: Splits CoordinatedPlatformLayerBuffer/ImageBackingStore implementations into TextureMapper-specific .cpp files and adjusts USE(TEXTURE_MAPPER) guards; isSameNativeImage inlined. Pure refactor/build reorganization.
This commit was flagged by the signal scorer and reviewed by AI (not a human). It is listed only so the triage is auditable; there is no root-cause analysis because this is not a vulnerability fix.

Structural signals: smartptr_member_added (+6.47), security_assert (+4.87), null_guard (+2.59), large_churn (-2.0)