Revert "SL-19331: No need to create texture in doMediaTexUpdate - should already be handled by preMediaTexUpdate"

This reverts commit bf9d0900.

SL-19513

SL-19331: Unlock mutex when bailing out from bad media state

SL-19331: Prevent some unnecessary image updates on media prims

SL-19331: Clarify that mipmapping on media prims being disabled is known behavior that should be corrected at some point

following promotion of DRTVWR-573

DRTVWR-559 Dynamic exposure followup -- stochastic sampling, weight based on luminance and distance to center of screen, rebalance night, don't rely on blending not clamping R16F.

SL-19331: No need to create texture in doMediaTexUpdate - should already be handled by preMediaTexUpdate

SL-18563

DRTVWR-559 Fix for off-white metal not looking chromed when it should (fake hacky HDR sky reflections).

SL-18563