Add GL_EXT_relaxed_block_layout

[Karpov-Ruslan]

This extension is written to support VK_KHR_relaxed_block_layout Vulkan extension. Currently there is only VK_EXT_scalar_block_layout support - GL_EXT_scalar_block_layout. To correct this historical injustice, I present this extension.

If GLSL Working Group approves this extension, then I will create a PR in glslang (link to fully working fork with the extension implementation).

The example of usage GL_EXT_relaxed_block_layout:

#version 460 core

#extension GL_EXT_relaxed_block_layout : require

struct S {
    int i;  // Struct member offset: 0
    vec3 j; // Struct member offset: 4
};

layout(binding = 0, std430, relaxed) buffer SSBO {
    int a; // Offset: 0
    S s;   // Offset: 16 (the base alignment of structure equals to 16)
    int b; // Offset: 32 (the size of structure equals to 16)
};

void main() {...}

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