Don't rotate the logo on the loadscreen.

That was some test code committed by mistake.
Add a separate cache for brightened images.
Do set the color mod on the actual texture.
Use the correct cache for non-hexed images.
Store alpha value in ttexture and set it before drawing.

Several ttexture instances may share a single SDL_Texture, so having
everyone store their alpha mod directly in the SDL_Texture object is not
viable.
Remove set_blend_mode from ttexture.

I can't think of any use case when we would use anything else than
SDL_BLENDMODE_BLEND. If anyone can, they can re-add it.
Initialize texture alpha mod to 255 instead of 0.
Set the color mod before rendering.
Return an int for the width/height of ttexture.

SDL (and all of our own functions) use ints, so having these functions
return an unsigned value can lead to all kinds of two's complement
troubles.
When retrieving dimensions, return the scaled size of the texture.
Render background layers as textures on the storyscreen.
Render floating images as textures on the storyscreen.
Match function declarations with definitions.
Enable rendering text as a texture.
Render everything on the loadscreen using the renderer API.

Only when building against SDL2, obviously.