Add support for the `glGetActiveUniformBlock` family of functions

[mbrla0]

Fixes #183.

What is it?

This pull request adds support for the glGetActiveUniformBlock family of functions through the addition of three new functions to HasContext:

• get_active_uniform_block_parameter_i32, which retrieves a single signed integer parameter and is intended for use with the following pnames:
  • GL_UNIFORM_BLOCK_BINDING,
  • GL_UNIFORM_BLOCK_DATA_SIZE,
  • GL_UNIFORM_BLOCK_ACTIVE_UNIFORMS,
  • GL_UNIFORM_BLOCK_REFERENCED_BY_VERTEX_SHADER, and
  • GL_UNIFORM_BLOCK_REFERENCED_BY_FRAGMENT_SHADER.
• get_active_uniform_block_parameter_i32_slice, which retrieves a slice of signed integer parameters and is intended for use with the following pname:
  • GL_UNIFORM_BLOCK_ACTIVE_UNIFORM_INDICES.
• get_active_uniform_block_name, which retrieves the name of a uniform block, as a string.

Potential Implementation Pitfalls

In the WebGL backend, the integer data types returned by the getActiveUniformBlockParameter function are unsigned, rather than signed, as they are in OpenGL. In my implementation I have decided to follow OpenGL, rather than WebGL, and have thus opted to implement functions with an i32 return type, rather than with a u32.

When retrieving the name of an active uniform block in the native backend, this implementation will buffer 256 bytes on the stack and pass those as the receiving name buffer to OpenGL, as there doesn't seem to be a way to query for the length of the uniform block's name ahead of time.

[mbrla0]

Could we use GL_UNIFORM_BLOCK_NAME_LENGTH in glGetActiveUniformBlockiv? 256 seems like a reasonable default if we can't use that for some reason.

Oh, totally. I missed that parameter as I was mostly only reading the WebGL documentation, which doesn't have it. My bad.

[bors[bot]]

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