SL-19538 Followup -- tune exposure parameters and clamp local light ambiance.  Make render targets 16F and scrube NaNs (thanks Rye).  Update midday. (#154)

* SL-19331: Have media updates on the LLImageGL thread even when texture updates are on the main thread. Add config.

Off-thread media updates seem work best performance-wise when using glTexImage2D, not sub_image_lines. Otherwise, there are lots of main thread stalls.

* SL-19331: Bump featuretable

* SL-19331: Cleanup, annotate comment

SL-19331: Partially revert code trim in LLViewerMediaImpl

* SL-19538 Remove hacky ambiance scale and take the mittens off probe ambiance values.  Fix for sky brightening being done in sRGB space.

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