Fix compute on Vulkan when there is only one buffer but no uniforms (#2359)

When using this compute shader, the following validation errors appear.
This patch fixes the issue.

BUFFER_WR(cBuffer, uint, 1);

NUM_THREADS(1u, 1u, 1u)
void main()
{
	cBuffer[0] = 0u;
}

../../../src/renderer_vk.cpp (628): BGFX ---E-       CommandBuffer,
Validation, 0: Validation Error: [
VUID-vkCmdBindDescriptorSets-dynamicOffsetCount-00359 ] Object 0: handle =
0x7fffe453ec88, type = VK_OBJECT_TYPE_COMMAND_BUFFER; | MessageID =
0x82756c54 | vkCmdBindDescriptorSets(): Attempting to bind 1 descriptorSets
with 0 dynamic descriptors, but dynamicOffsetCount is 1. It should exactly
match the number of dynamic descriptors. The Vulkan spec states:
dynamicOffsetCount must be equal to the total number of dynamic descriptors
in pDescriptorSets
(https://www.khronos.org/registry/vulkan/specs/1.2-extensions/html/
vkspec.html#VUID-vkCmdBindDescriptorSets-dynamicOffsetCount-00359)

../../../src/renderer_vk.cpp (628): BGFX ---E-            Pipeline,
Validation, 0: Validation Error: [ VUID-vkCmdDispatch-None-02697 ] Object 0:
handle = 0xcd00000000cd, type = VK_OBJECT_TYPE_PIPELINE; Object 1: handle =
0x630000000063, type = VK_OBJECT_TYPE_PIPELINE_LAYOUT; Object 2:
VK_NULL_HANDLE, type = VK_OBJECT_TYPE_PIPELINE_LAYOUT; | MessageID =
0xfd9e3152 | vkCmdDispatch(): VkPipeline 0xcd00000000cd[] defined with
VkPipelineLayout 0x630000000063[] is not compatible for maximum set
statically used 0 with bound descriptor sets, last bound with
VkPipelineLayout 0x0[] The Vulkan spec states: For each set n that is
statically used by the VkPipeline bound to the pipeline bind point used by
this command, a descriptor set must have been bound to n at the same
pipeline bind point, with a VkPipelineLayout that is compatible for set n,
with the VkPipelineLayout used to create the current VkPipeline, as
described in Pipeline Layout Compatibility
(https://www.khronos.org/registry/vulkan/specs/1.2-extensions/html/
vkspec.html#VUID-vkCmdDispatch-None-02697)

../../../src/renderer_vk.cpp (628): BGFX ---E-       CommandBuffer,
Validation, 0: Validation Error: [
UNASSIGNED-CoreValidation-DrawState-DescriptorSetNotBound ] Object 0: handle
= 0x7fffe453ec88, type = VK_OBJECT_TYPE_COMMAND_BUFFER; | MessageID =
0xcde11083 | VkPipeline 0xcd00000000cd[] uses set #0 but that set is not
bound.

src/renderer_vk.cpp

@@ -6808,6 +6808,7 @@ VK_DESTROY
 							allocDescriptorSet(program, renderBind, scratchBuffer);
 						}
 
+						uint32_t numOffset = 0;
 						uint32_t offset = 0;
 
 						if (constantsChanged
@@ -6816,11 +6817,15 @@ VK_DESTROY
 							const uint32_t align = uint32_t(m_deviceProperties.limits.minUniformBufferOffsetAlignment);
 							const uint32_t vsize = bx::strideAlign(program.m_vsh->m_size, align);
 
-							offset = scratchBuffer.m_pos;
+							if (vsize > 0)
+							{
+								offset = scratchBuffer.m_pos;
+								++numOffset;
 
-							bx::memCopy(&scratchBuffer.m_data[scratchBuffer.m_pos], m_vsScratch, program.m_vsh->m_size);
+								bx::memCopy(&scratchBuffer.m_data[scratchBuffer.m_pos], m_vsScratch, program.m_vsh->m_size);
 
-							scratchBuffer.m_pos += vsize;
+								scratchBuffer.m_pos += vsize;
+							}
 						}
 
 						vkCmdBindDescriptorSets(
@@ -6830,7 +6835,7 @@ VK_DESTROY
 							, 0
 							, 1
 							, &scratchBuffer.getCurrentDS()
-							, constantsChanged || hasPredefined ? 1 : 0
+							, numOffset
 							, &offset
 							);
 					}