Feature(v2) texture compression

[jfboeve]

Reworked the texture compression feature.

• fixed ktx loading when sizes are not perfectly divedable by format block sizes.
• cleaned up pvr data loading / uploading to webgl.
• added astc support (possibly not usable on most devices at this point in time).
• compressed textures now properly fire loaded and failed events.

[michielvandergeest]

this PR also adds support for ASTC textures and closes #610

[michielvandergeest]

LGTM!