Use texture swizzle to implement common unsupported GLES texture format

[riccardobl]

When we switched the default to ANGLE_GLES we lost the following common texture formats

Format.BGR8
Format.ARGB8
Format.BGRA8
Format.ABGR8

both linear and sRGB.

Jme supports them on the GL path, so games wishing to switch to Angle GLES might experience this breaking change.
We should reintroduce the support for those formats using texture swizzle, both for srgb and linear images.