spearmint: Fix fading Q3 flare shader

Q3 and RTCW/ET flare shaders use different blend modes, need to support both
in tr_flares.c. Q3 flare fades color, RTCW/ET flare fades alpha.

Also let's not hack the shader's adjustColorsForFog, use fogNum 0 instead.
Also respect the shader 'noFog' option that is used by RTCW/ET.
spearmint: Remove the flare shader hack to disable depth test

It's probably an optimization, but let's kill it and maybe readd hack into RB_RenderFlare later.

It is possible (in Spearmint and FAKK) to specify 'noDepthTest' in the shader.
spearmint: Allow specifying shader for corona

Needed for RTCW SP, which uses two corona shaders.
Also just a good idea in general.

RTCW SP API uses a bit flag to specific flare or stoplight,
but let's take shader handle instead.