Feature: Introduce writeSubData in Texture API

[nemerle]

This is mostly a proposal and a call for help from D3D11 and Vulkan gurus out there.

What is missing:

• Vulkan support
• D3D11 support
• Optional written-area-within-target checks.

[BearishSun]

I'd prefer the new method is also named writeData and is just an overload. Other than that I think it's fine, as long as the underlying API's all support this.

[nemerle]

I'd prefer the new method is also named writeData and is just an overload. Other than that I think it's fine, as long as the underlying API's all support this.

I've originally named those methods writeData, but was unsure how well the @copydoc worked with overridden functions.