[RHI] Add sampler2DArray support

[halx99]

Describe your changes

• Add sampler2DArray in shader
• Remove texture array implementation in CPU side
• Simplify texture binding infos in ProgramState
• Add new API Texture2D::initWithSpec, all other init stubs will invoke it finally
• MipLevel improvemnts, remove API generateMipmaps in renderer, auto process in rhi
• Remove API Texture2D::setRenderTarget
• Rename Texture2D::getBackendTexture to Texture2D::getRHITexture()

Issue ticket number and link

Checklist before requesting a review

adopt changes

• FairyGUI etc1 image load
• metal backend
• mipmap logic
• dual sampler shaders
• ui::MediaPlayer
• AX_ENABLE_CACHE_TEXTURE_DATA
• Terrain
• RenderTexture

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• I have performed a self-review of my code.

  Optional:

  • I have checked readme and add important infos to this PR.
  • I have added/adapted some tests too.

For core/new feature PR

• I have checked readme and add important infos to this PR.
• I have added thorough tests.

Axmol 3.x ------------------------------------------------------------

For each 3.x PR

• Check the '#include "axmol.h"' and replace it with the needed headers.