spearmint: Support loading .ftx images in OpenGL2 renderer

.ftx is used by FAKK and Alice. It's already supported by OpenGL1 renderer.
spearmint: Have LIGHTMAP_NONE default to AGEN_SKIP

Implicit shaders and non-existant shaders using LIGHTMAP_NONE should use
AGEN_SKIP. CGEN_LIGHTING_DIFFUSE sets alpha to 255, so can skip AGEN_IDENITY.
spearmint: Use correct diffuse color gen for lightmap index

LIGHTMAP_NONE should use CGEN_LIGHTING_DIFFUSE, LIGHTMAP_2D should use CGEN_VERTEX,
others (for world) should use CGEN_EXACT_VERTEX. Like done in SetImplicitShaderStages.

This is required for FAKK and Alice as they use rgbGen lightingDiffuse on misc_models
(which can either be made static or dynamic; part of world or a separate entity).

The change to lightmap stage without lightmap should fix using lightmap
on entity models and 2D (though GetSurfaceShader overrides rgbGen anyway).