Change GL_RGB --> GL_RGBA8

[MegaMech]

This seems to be a spelling error since the other renderers use rgba8 or some variation of (bgra8).

This change will be necessary if the colour id framebuffer were to be implemented as it's based on a uint32_t instead of a 24 bit value.

[Archez]

These changes cause a significant difference in how OpenGL appears to render transparent textures: Example

The clouds on the map are darkened, the HUD A and C buttons are darkened. Water boxes, road paths, etc are all darkened.

This is how it looked before the changes: