Texture fixes
- refactoring of code responsible for texture uploading and attaching to a shader
- fixed appearance of black texture when texture has no mipmaps inside (for example, when all mips are in TFC and the TFC is missing) - that was caused by UTexture2D destructor which destroyed "default texture" when UTexture2D::TexNum was set to DefaultTexNum (that was fixed for UTexture, but not for UTexture2D - refactoring fixes that completely)
- disabled DXT5 format in Android/ETC data - this format maps to ETC2 which is not supported

Other
- fixed FArchive::Seek64() wasn't able to seek over 1Gb position (now limit is 2Gb)
- material viewer: disabled lighting when displaying a texture

Refactoring notes:
- Upload2D will return BAD_TEXTURE instead of DefaultTexNum when having troubles uploading texture data
- UUnrealMaterial: separated texture uploading code to 'bool Upload()' - returning 'false' when upload failed for some reason; Bind() returns bool too
- GL_NormalmapShader() now verifies all textures for correctness - it checks result of Bind() call and disables particular shader feature when texture is bad
- UUnrealMaterial::SetMaterial checks Bind() result too
- BindDefaultMaterial: part of code which creates a default texture is separated to GetDefaultTexNum(), so shaders could use "default texture" number when needed without being forced to call extra GL code from BindDefaultMaterial