Add more validation to UBO size and alignment in Compatibility renderer

[clayjohn]

This validation is to ensure we don't break the UBO alignment rules which can have disastrous performance implications on some buggy drivers.

This is mostly static checks to ensure we don't accidentally break things with subsequent changes. But it does add a check that the material buffer size does not exceed the max uniform buffer size.

The one substantial change here is that we change the validation for the global uniform buffer size. Previously it was MAXed to be at least 4096 items long and the raw size (number of items) was compared against the maximum UBO size (measured in bytes). So we still ended up requesting a buffer that was 16 times the maximum size supported by the device.

In practice, this hasn't resulted in any issues as Uniform buffers are allowed to be bigger than the device limit. The device limit specifies how much you can use in a draw call and the Compatibility renderer limits the number that can be used in a draw call to 256.

godot/drivers/gles3/rasterizer_canvas_gles3.cpp

Line 2796 in 505da68

global_defines += "#define MAX_GLOBAL_SHADER_UNIFORMS 256\n"; // TODO: this is arbitrary for now

At any rate, we need to fix this validation before users run into the problem (at best they will get cryptic driver errors, but they could easily get a crash)

Partially addresses: #85374

Putting on the docket for 4.3 as this is a very safe change and will potentially have benefits for web exports which struggle with large UBOs

[Calinou]

Code and docs look good to me.

[akien-mga]

Thanks!

[patwork]

@clayjohn hi, now every time I run Godot in compability mode I get a warning:

WARNING: Project setting "rendering/limits/global_shader_variables/buffer_size" exceeds maximum uniform buffer size of: 4096. Falling back on maximum buffer size.
     at: MaterialStorage (drivers/gles3/storage/material_storage.cpp:1121)

godot/drivers/gles3/storage/material_storage.cpp

Lines 1110 to 1116 in b75f048

	static_assert(sizeof(GlobalShaderUniforms::Value) == 16);
	global_shader_uniforms.buffer_size = MAX(16, (int)GLOBAL_GET("rendering/limits/global_shader_variables/buffer_size"));
	if (global_shader_uniforms.buffer_size * sizeof(GlobalShaderUniforms::Value) > uint32_t(Config::get_singleton()->max_uniform_buffer_size)) {
	global_shader_uniforms.buffer_size = uint32_t(Config::get_singleton()->max_uniform_buffer_size) / sizeof(GlobalShaderUniforms::Value);
	WARN_PRINT("Project setting \"rendering/limits/global_shader_variables/buffer_size\" exceeds maximum uniform buffer size of: " + itos(Config::get_singleton()->max_uniform_buffer_size / sizeof(GlobalShaderUniforms::Value)) + ". Falling back on maximum buffer size.");
	}

• GLOBAL_GET("rendering/limits/global_shader_variables/buffer_size") is by default 65536 (clean config)
• sizeof(GlobalShaderUniforms::Value) is 16
• Config::get_singleton()->max_uniform_buffer_size is 65536 on my system (windows 10, nvidia gtx 960)

So in line 1113 there's check if (65536 * 16 > 65536) and it's always true