This fixes a potential crash in OpenGL when changing between levels.

`renderdeltatics` can be used as a standard delta time in any place,
allowing for smooth menus. It will always be equal to `realtics`
when frame interpolation is turned off, producing consistent
framerate behavior everywhere it is used.

This seems like an appropriate way to describe the feature for now.

Instead of interpolating thinkers, we interpolate mobjs inside the
renderer. Further interpolation is TBI.

Fix frame blend flags not working with 3D models

See merge request !1778

Fix papersprites being rotated incorrectly in Software when viewing them on the flipped side

See merge request !1770

Add angle field to linedef_t

See merge request !1602

Prevent Hangsters from sliding across the ceiling

Closes #787

See merge request !1766

Fix Ideya Capture sometimes stealing extra spheres

See merge request !1768

Do not reload every texture when adding a wad

See merge request !1725

Drop shadow fake floor/ceiling fix

See merge request !1638

Fix Minus & debris' reverse gravity behavior

See merge request !1764

Show overall grade emblem requirement of a multi-mare NiGHTS stage in SP pause menu

Closes #71

See merge request !1762

Remove drop shadow from badnik explosions, give drop shadows to Metal and flickies

Closes #218

See merge request !1761

Make S_IVSP and  S_THUNDERCOIN_SPARK fullbright

See merge request !1755

Make P_SpawnGhostMobj ghosts properly copy spritestuff2 (!1257) variables

See merge request !1741

Level platter name improvements

See merge request !1730

Also clean up redundant "startingspheres" variable

Fix typo in linux-only compilation flag

See merge request STJr/SRB2!1759

Remove MODID check from hooklib to fix compile issue (resolves #809)

See merge request STJr/SRB2!1758

Fixes #71

- Added A_SetShadowScale
- Added A_ShadowScream which combines the above with A_Scream, used for 
badnik explosions
- Gave flickies drop shadows